One key element is the contrast. You have the cute and friendly world of My Little Pony and then you introduce the creepy Slender Man. It creates a really interesting dynamic.
There's a fanfic called 'Among the Ponies, He Stalks'. In this story, Slender Man's presence starts to cause chaos in Equestria. It's fascinating to see how the different pony characters, with their distinct personalities, deal with the fear and mystery that he brings. Some ponies try to face him bravely, while others are paralyzed by fear, making it a very dynamic read.
There are likely fanfics where the mlp characters have to face the psychological terror that Slender Man usually brings. In these stories, the art style of My Little Pony might be blended with the dark and shadowy look of Slender Man. Some fanfics could explore how the different pony races, like Pegasi or Unicorns, react to the presence of Slender Man, whether they use their flight or magic to try and combat this otherworldly entity.
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.
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.
The mlp slender man fanfic genre blends the two concepts by taking the cutesy, colorful world of My Little Pony and injecting the dark, creepy elements of Slender Man. For instance, it might put the ponies in situations where they are being stalked by Slender Man in their own familiar settings like Canterlot or Sweet Apple Acres.
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.