Yes, in some fanfictions, Harry is depicted as the son of Slender Man. Fanfiction often takes creative liberties and creates unique relationships between characters.
A plot could involve Slender Man corrupting some of the magical creatures in the Harry Potter universe. For example, he could turn some of the centaurs or thestrals into his minions. Harry and Hermione would start researching about this strange new entity in the library. Ron, on the other hand, would be more skeptical at first. But as the situation worsens, they all team up. They might seek help from Hagrid who has knowledge of the more unusual creatures and together figure out a way to stop Slender Man from spreading his influence further.
Neville would be terrified. Having faced his own fears in the wizarding world, Slender Man's appearance would bring back all those insecurities. Ginny, on the other hand, would be more defiant. She has had her own experiences with dark forces and would not be easily intimidated. The Weasley twins might see it as an opportunity for some new pranks, though they would quickly realize the seriousness of the situation. Hagrid would be confused as he has never seen such a strange creature in the Forbidden Forest. He might try to approach Slender Man in a friendly way at first, which could be a dangerous mistake.
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.
One common plotline is Harry trying to hide his identity. Since Slender Man is often seen as a menacing figure, Harry doesn't want others to know his relation.
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.
In traditional Harry Potter fanfictions, Harry is usually raised within the wizarding world norms, like by his parents or the Dursleys. But in 'Harry Potter Raised Slender' fanfictions, his upbringing is by Slender, which is a very different entity. This changes the entire basis of his character development and the plot. For example, his view on magic and the people around him can be drastically different.
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.