One possible plot could be that Ochaco wakes up one day as a kid and has to figure out how to reverse it while also dealing with her now child - like body. She might struggle with her hero training as a kid, like not being able to reach high places or lift heavy objects like before. And she could have cute interactions with her classmates who are now much bigger and older - looking compared to her.
Mentally, Ochaco would struggle. She has the mind of an older person but is trapped in a child's body. She would have to deal with the frustration of not being able to communicate as effectively as she used to. And she might feel a sense of loss for her normal life and the progress she had made in her hero training and relationships.
If Deadpool is turned into a kid in fanfiction, it would be a total shift in his character dynamic. His relationship with other characters would change drastically. For example, his friends might have to take on a more parental role. And his enemies might not take him as seriously at first, but he could still surprise them with his child - like cunning. Also, his origin story would be interesting to re - imagine from a kid's perspective, how he discovers his powers and all that.
Well, in such a fanfiction, Sonic might lose his usual speed and confidence at first. He'd probably be more naive and rely on his friends more. Maybe he'd have to learn basic things all over again, like how to properly control his powers in his new, smaller form.
Well, he might lose all his evil cunning at first. Since he's a kid, he could start to see the world in a more innocent way. Maybe he'll make friends with people he used to terrorize, not realizing who they are. And he could be easily influenced by good-hearted characters, starting to develop a sense of right and wrong that's completely different from his adult, villainous self.
Scott would probably be very protective. He'd feel responsible for taking care of Stiles and making sure he doesn't get into too much trouble. Lydia might be initially shocked but then use her intelligence to try and figure out how to reverse the situation. And Derek, well, he'd probably be a bit annoyed at first but would also end up being one of Stiles' protectors.
In some fanfictions, a possible plot could be that Stiles, as a kid, has a different perspective on the supernatural world around him. He might not be as afraid as he was when he was older. For example, he could befriend a supernatural creature that the older Stiles would have been more cautious about. This new friendship could lead to new alliances for the pack. Also, his family's reaction to his sudden transformation could be explored. His dad might be really confused and worried, while his friends are trying to keep it a secret from the wrong people in town.
It could also be a new villain. Maybe someone who has been waiting for Sonic to be in a vulnerable state. This new villain could have a special power or weapon that is more effective against Sonic as a kid. For example, a device that can slow down Sonic even more or nullify his powers temporarily. They might target Sonic not just because of his weakened state, but also because they know that his friends will be distracted trying to help him get back to normal.
Often, there's a plot where Ron, as a kid, has to deal with bullies. Since he's smaller and less experienced, it makes the situation more challenging. He might then learn some valuable lessons about standing up for himself. Also, there could be a plot where he discovers a hidden talent that he didn't know he had as an adult, like being an amazing artist or a natural at flying on a broomstick even at a young age.
The other villains in Gotham. The power dynamic changes when the Joker is a kid. They might see this as an opportunity to take over his territory or try to use the situation to their advantage. But they also might be worried that if the Joker turns back to normal, he'll come after them for taking advantage of his weakened state.