(A.N - I look up at the age of Itachi when the Kyuubi attack and it said that Itachi is 5 which doesn't make sense when he is born year 59 and I make the age corresponding to the year till year 65 which is the year the Kyuubi attack.
Year 59 - 0 Y.O(Itachi and Shinsei were born and I think when you were born you became automatically 1 year old but I make it 0 years old)
Year 60 - 1 Y.O(Tsuyoshi born, Itachi and Shinsei become 1 Y.O)
Year 61 - 2 Y.O(Shinsei and Itachi 2 Y.O, Tsuyoshi 1 Y.O, Itachi's younger sister born(don't know the name yet)
Year 62 - 3 Y.O(Tsuyoshi 2 Y.O, Itachi's sister 1 Y.O)
Year 63 - 4 Y.O
Year 64 - 5 Y.O
Year 65 - 6 Y.O(since Itachi is born June 9 and Shinsei is September 14, while the Kyuubi attack is October 10 which is the year naruto is born)
So yeah it doesn't make sense for Itachi and Shinsei to be 5 years old when the Kyuubi attack, so I will make it to 6 years old or heck they might be 7 Y.O by now if the day they are born is included to be 1 Y.O)
4 years later, in Year 64, The war ended, and the Third Hokage, Hiruzen announced that he will step down as a Hokage and pass it to Minato also known as Yellow Flash, Many things happened in the 4 years time skip,
Year 61,
Tsuyoshi became 1 year old, and Mikoto gave birth to her second child which is a girl, she is pregnant, Year 60, November 26 and gave birth on July 27, Year 61 (obviously, this is for word count), and Sei congratulate Mikoto in giving birth to her second child with Shinsei, Tsuyoshi and Itachi who is happy having his blood-related sibling, Shindo also got back last month June 10, for the birthday of his second son and he also gets to know Mikoto with Itachi when she visits them, to their house and congratulate her in being pregnant. He also promise that he will be back 4 months later for his 1st son's birthday, and he did fulfil his promise and go back the day after the birth.
Year 62,
Itachi and Shinsei become 3 years old and the same thing happen last year, except for the birth of Mikoto of course, Shindo going back and forth for the birthday of his sons, and he(Shindo) also got to know Fugaku which he became friends with when they are drinking at the birthday of his second son which his age is 2 y.o and Mikoto's daughter is one year old.
Year 63
The same thing happens again being Shindo always at their birthday and this time he attended Itachi's birthday which is June 09, and his Friend Fugaku kept bragging about his son being a genius, well, of course, Shindo too bragged about his son being a genius, when he tested with chakra paper, Itachi's chakra nature is fire being his natural element and water being his secondary, While mc has four being Earth, Lightning, Yin(reincarnation), and Yang, which Shinsei acquired some knowledge of jutsu for each type of chakra nature.
Year 64
The war has finished, and the third Hokage announced the retirement of his position as the Hokage and passed it down to Minato also known as Yellow Flash, Shindo also got home to see his family, with some scar on his cheek which he got from the war which adds to his manliness, Tsuyoshi and Itachi's sister is always close that don't want to be separated with each other, while Itachi and Shinsei are already 5 years old.
Somewhere around the forest(copy-paste from the fight scene)
Shinsei and Itachi face-off of each other, staring, while the atmosphere in the forest is heavy, Shin looked at Itachi then rush towards him readying his punch, Itachi follows to punch Shinsei whose punch is only faint while dodging to the side and grabbing the arm of Itachi.
Itachi dodge the grabbing hand of Shinsei while doing a side step and sweep his leg towards Shinsei while Shinsei jumps to avoid the sweeping leg Itachi and kick Itachi mid-air who dodge the kick, Shinsei then proceeds to back off facing Itachi who do the same, they rush to each other punching the face of Itachi who twist his face to minimize the damage while he follows up a left hook towards Shinsei's liver who covered it with right arm.
Shinsei then kicks Itachi in the stomach who fly back to the tree, he(Itachi) black flip and kick the tree for balance, Shinsei See's this dash towards him and kicks him to the head who dodge it by leaning his head with his body backwards, Shinsei follows up with an axe kick towards Itachi, who tries block it but can't completely block since his body is bent, he got to the ground coughing some saliva, Shinsei then smiles and help Itachi get up.
Shin: "I win"
Itachi: "Yeah,yeah*coughing* be gentle next time, you almost hurt my back"
Shinsei already sense someone in the midway of their battle which he ended quickly.
Shin: "Who's there, show yourself!"
Itachi: "What are you shouting for, Shin"
Shin: "Someone is here, watching us when we are sparring"
Shisui who's watching them from the top of the tree, was shocked when Shinsei sense him since he is hidden well and hid his presence, Shinsei should not be able to detect him since they are 5 y.o and they shouldn't have learned that much in 1 year when their chakra coil (or is it pathway) is unlocked.
Shinsei looks at the tree in front pick a stone from the ground and throw it to the tree in front of them. Shisui sense an object coming towards him which he dodge and look at the hole which the stone pass through, he was shocked at the prowess Shinsei used, Shisui then walks out of the forest and confront him Shinsei and Itachi.
Shinsei and Itachi looked at each other and then at Shisui, while Shinsei already guesses who he is, but he feign ignorance of shisui while Itachi warily looked at Shisui who has a sword on his back.
Shisui: "I'm not a suspicious guy, my name is Shisui Uchiha by the way nice to meet you"
Itachi was surprised to see an Uchiha member in this forest, he looked at Shinsei who also looks surprised by the word of Shisui(not really), Itachi then calms down and calmly spoke.
Itachi: "So you are an Uchiha huh, I'm Itachi Uchiha by the way and this is Shinsei my best friend"
Shisui: "What!? so you are Itachi Uchiha the son of the clan head
Shinsei: "Well you already know my name but I will introduce myself again, I'm Shinsei and why are you watching us by the way?"
Shisui: "Oh I was just practising my shuriken and kunai throw, at the forest at the south fifty meters away from here" Shisui pointed behind his back which he came from.
Shisui: "I heard a noise here and run to see what's going on and see you sparring with each other, so I watched and the rest is history"
Shinsei: "Sorry about that, I thought someone is stalking us" while scratching his head while looking away and whistling.
Shisui: "Ah, don't worry about it, by the way, how did you sense me, I hid well you know"
Itachi: "Don't overthink of that monster, he's good at everything when we are little"
Shinsei: "What do you mean monster, you're just jealous cause I always win against you" while sticking his tongue out(😛 like this) and running while Itachi chase after him while throwing stones towards him which he dodges or catch and throw it back.
Shisui looks at the two who seems to be secretly playing tag with the guise of chasing after Shinsei, Shisui just smiles and sat at the boulder near him watching the two chasing around while Shinsei does the killua style when he backflips and he smashes the icing(or is it cake?) to gon but only the backflip and kick Itachi at the back lightly.
Minutes later, Itachi already run out of breath while Shinsei is still running around Itachi in a circle, Itachi walk towards Shisui and sat down while Shinsei follows cautiously towards Itachi to get ready for the ambush, seeing that Itachi just sat down he dropped his guard and sit next to Itachi.
Shisui: "So what are you guys gonna do now?"
Shinsei: "Who knows, we're just sparring and then you arrive, so we don't know what to do"
Shisui: "Why don't you follow me to my training ground for shurikens and kunais"
Shinsei: "Sure that would be cool, what do you think, Itachi"
Itachi: "Hmm, alright since we got nothing to do anyway and it's still 1 pm(look towards the sun to measure what hour it is) anyway we have free time till evening"
Shinsei and Itachi follow Shisui towards his training ground for shuriken and kunai throwing, they arrived seconds later to see many metal plates around the tree.
Shisui: "This is my training ground of when I practice shuriken and kunai"
Shinsei: "Cool, so this is your training ground huh"
Itachi: "Hmm"
Shinsei: "Well, come I'll teach you how to throw shuriken and kunai properly, so who's first"
Shinsei: "You go first Itachi, I will watch you so don't you dare fail throwing"
Itachi: "Cheh, you just want me to embarrass myself"
Shinsei: "Hehehe, I will laugh at you, you know, since you're my best friend and what best friend are they if they laugh at their friend"
Itachi: "You are just talking to yourself idiot"
Shisui just watches and wait for them to stop bickering.
Shinsei: "Now now, just practice your throwing skill since Shisui is waiting"
Shisui and Itachi then practice one shuriken throw while Itachi easily learns from Shisui how to throw it properly, hitting the centre of the plate, Itachi smile.
Shinsei: "Now it's my turn, can I borrow your shuriken, Shisui"
Shisui: "Here" he passes a shuriken to Shinsei.
Shisui: "but do you know how to properly throw the shuriken?"
Shin: "I know since the time you teach Itachi how to properly throw it, don't worry I have a good memory"(Instant learn)
Shisui: "Okay"
Shisui and Itachi just watch Shinsei throw the shuriken hitting the centre, Shisui just stares at Shinsei a little shocked by how Shinsei perfectly hit the plate at the centre.
They continue to practice their throwing while running and of course, Shinsei always hit the centre, 4 pm arrived and they split with each other when they arrive at the playground. Itachi and Shisui walk with each other while Shinsei walked to his house thinking about what will happen in a year, the event where the seal which Kyuubi is weakening from kushina's stomach and being released by Obito.
And what he will do in the future does he save them from being killed so that the incompetent hiruzen will not be a Hokage again or just let them die and make Naruto's childhood hell, of course, they cannot adopt naruto since he is a jinchuriki, that means he only has one option and that is helping the 4th Hokage wife to safely birth.
He plans on what to do when the Kyuubi is released, he can undo the genjutsu and put his genjutsu to the Kyuubi, while he split it up so that naruto can have the yang Kyuubi to him and yin Kyuubi to Minato, and save Kushina from dying because of the Kyuubi.
Or he can just dump the full Kyuubi towards naruto or Minato then heal Kushina.
He also plans on what to do with Obito will he let him get away or not, of course, he will not let him get away, he will put a powerful fuinjutsu that can nullify the power of MS or EMS, and give him to Obito's, that means he needs to plan where to put the fuinjutsu is and that can make it invisible to anyone and making it complex so that they can't decode it.
While he kept thinking about his plans he arrived at his house, knocking at the door, the door open and his Father is seen with his cheek having an x slash.
Shindo: "Oh, here's the little guy, come inside or your mother will be angry at you" Shindo hold Shinsei's hand while guiding him to the living room where his mother and brother is.
Shinsei: "Good evening mom, hello little brother"
Tsuyoshi: "Good evening brother Shin"
Sei: "So arrived huh, so what are you doing the afternoon"
Shin: "Just sparring with Itachi and I also got a new friend, his name is Shisui Uchiha"
Sei: "Oh an Uchiha huh, you always have a friend in Uchiha household but not the other clans or civilians"
Shin: "Well, I'm always with Itachi and since Itachi has no friends other than me, I accompany him and it's not like I can't make friends at the academy when I go next year, we talked about early graduation"
Sei: "So you want to be a shinobi huh, well it's your choice anyway and I will talk to Mikoto about this early graduation, since Mikoto is pregnant again(November 13, Year 64 and she will give birth to Sasuke on July 23) I will check on her tomorrow."
Shinsei: "Oh, so Aunt Mikoto is having a baby again, by the way, mom, dad how do you make a baby?" Shin asked innocently while laughing inside.
While Tsuyoshi hears the question of his brother also asked and stares at their mother and father.
Sei and Shindo with Tsuyoshi stare at them, stare at Shinsei and Tsuyoshi, while a little surprised and embarrassed because of the question of Shinsei.
Sei: "You wait till you get older and you will eventually know it, and you too Tsuyoshi"
Tsuyoshi: "Okay, by the way, mom when will we go meet with aunt Mikoto and _______(Itachi's sister), cause I want to meet her"
Sei: "Tomorrow, now I will cook some food while Shindo, you take care of the kids "
Shindo: "I will honey, come to my side Shin"
Shinsei then walks to his father and sit down while talking about what happened today like Itachi's defeat, the throwing practice and making a new friend.
Shindo: "Good good good, that's my boy, I will tell Fugaku about his son being defeated by my son, hahaha" Shindo laugh proudly at his son's sparring and winning.
Minutes later the food arrived and was ready to serve at the table, Sei shouted towards them at the kitchen that the food was ready, they got up and walk towards the kitchen where the food is, they eat until they are full and sleep.