The child could be an orphan from a far - off land that Percy visited. Maybe Percy found the child alone and starving, and felt responsible for the child's well - being, so he brought the child back.
The child might be a young demigod that Percy discovered during his travels. Since Percy has experience as a demigod himself, he decides to bring the child back to a place where the child can be trained and protected, and where the child can find others like them.
Well, one possibility could be that Percy has been on some sort of adventure and during that time, he discovers a child in need. He decides to take the child under his wing and brings the child back to his world. Then, he has to deal with the reactions of his friends and family, who are both surprised and a bit worried about this new addition.
The child could be a prophesied one. In the world of Percy Jackson, prophecies are common. So, this child could be the one foretold in some ancient prophecy that is linked to Tartarus. Percy finding the child sets off a chain of events that will either save or doom the world. Maybe the child is the key to closing a portal in Tartarus that is releasing evil into the mortal world, and Percy is the only one who can help the child fulfill this great task.
The child could be a young demigod who got lost on a quest and ended up in Tartarus. Maybe they were trying to prove themselves and got in over their head.
It's possible that the child is an orphan Naruto found. Naruto, having known the pain of being alone as a child, decides to take the orphan under his wing. He brings the child back to Konoha to give it a home and a family within the village.
Perhaps the child is the offspring of a Titan. This would make for an interesting story as Percy would have to deal with the potential danger and mistrust from other demigods. But Percy, being Percy, would see the good in the child and adopt them regardless. The child might have unique powers that set them apart from other demigods, like the ability to control time or space.
It could be a child of a minor god who has been overlooked. Maybe the child of Hecate, a young demigod with some really interesting magic powers that Percy and Artemis see potential in.
One possible plot could be that Naruto has been on a long and secret mission. During this time, he met someone and had a child. Now he returns to the village with the child, and the villagers are both shocked and curious. His friends, like Sasuke, might be initially skeptical but then come to accept the new situation. The child could also have some unique powers inherited from Naruto, which could lead to various adventures as they try to figure out how to control and develop these powers.
Maybe it could be a new character. A mysterious woman who Percy meets during one of his adventures. This would add an element of mystery to the story.
In this fanfiction, perhaps the child is Naruto's own. Maybe he had an adventure away from Konoha and met someone special. The child could be a symbol of his growth and newfound family. He returns to Konoha with the child to introduce it to his friends and show how he has changed.
It could be a child of one of Naruto's friends who passed away. For example, if Shikamaru died in a battle, Naruto might take in his child. Naruto would then try to pass on Shikamaru's wisdom and strategic thinking to the child, along with his own ninja skills.