I'm not sure about its popularity. Fanfictions can vary greatly in how well - known they are. It depends on how many people are interested in the crossover concept of Naruto and Slender Man. There are so many fanfictions out there that it's hard to say if this one has a large following.
In this fanfiction, it might start with a strange backstory of how Naruto came to be the son of Slender Man. Maybe there was some sort of otherworldly interference. His relationships with his friends like Sasuke and Sakura would be very different. They might be scared or intrigued by his new heritage. And Naruto himself would have to deal with the fact that he's not what he thought he was, all while facing the usual challenges in the Naruto universe, like fighting enemies and protecting his village.
Maybe in the fanfiction, Slender Man has an alliance with Orochimaru. Orochimaru is always interested in strange and powerful beings. Together, they plan to take over the ninja world. Naruto and his friends then have to not only face Orochimaru's usual schemes but also deal with the terrifying abilities of Slender Man.
No, the Slender Man is not real. It was created as a fictional character in an online Photoshop contest. But the stories and the idea of him have had a real impact on people, especially in some disturbing cases where people were influenced by the concept and did bad things.
Slender Man child fanfiction differs mainly in the perspective. Regular fanfiction might focus on adult protagonists dealing with Slender Man in a more 'grown - up' way, like through investigations or complex psychological battles. In child fanfiction, it's about how kids experience the horror. It's more innocent yet also scarier in a sense as children are more vulnerable.
Bart would probably be both scared and curious. He might initially try to play pranks on Slender Man, but as things got scarier, he'd realize he was in over his head. Lisa, on the other hand, would likely try to research Slender Man to find a scientific or logical explanation for his existence.
Sasuke would likely be on high alert. He'd immediately analyze Slender Man's movements and try to find a weakness. He might start by using his Sharingan to see if he can detect any patterns in Slender Man's attacks.
Ruby would probably be both curious and cautious. Her adventurous spirit might make her want to learn more about this strange new entity, but her sense of responsibility for her team would keep her from being too reckless. Weiss would likely be initially dismissive, thinking it's some kind of trick or a new enemy that can be easily defeated with her skills and Dust magic. Blake would be more on guard, her past experiences making her more aware of hidden threats. Yang would probably be the first to want to throw a punch, seeing Slender Man as a direct threat to her and her sister.
One possible scenario in this fanfiction could be that Slender Man, instead of his usual menacing self, shows a strange sort of protectiveness towards the child. Maybe he sees the child as an innocent being in a cruel world. However, this goes against his typical portrayal as a terrifying entity. It could be an interesting exploration of his character, seeing a different side of him.
Yes, in some fanfictions, Harry is depicted as the son of Slender Man. Fanfiction often takes creative liberties and creates unique relationships between characters.