Well, in a fanfiction, it could be due to some sort of magic spell gone wrong. Maybe a villain was trying to defeat Spiderman in a new and unique way and accidentally turned him into a baby. Another possibility is that some strange scientific experiment in the Marvel universe backfired on him.
His identity might be at risk. If people find out that Spiderman has turned into a baby, it could expose his secret identity. Also, his friends and family would have to take on new roles. For example, Aunt May would have to be more of a caretaker than ever, and his superhero friends would need to protect him from villains who might see this as an opportunity to get rid of Spiderman once and for all.
One idea could be that in this fanfiction, Wolverine turns into a baby due to a powerful mutant's accidental spell. The X - Men then have to take care of him while also trying to find a way to reverse the transformation. They might discover new aspects of Wolverine's personality as a baby, like his unwavering determination is now shown through his attempts to crawl towards danger.
One idea could be that a villain's new invention accidentally zaps Kim and turns her into a baby. Her sidekick Ron has to take care of her while also trying to find a way to reverse the effect. He might struggle with basic baby - care tasks like changing diapers and feeding her, all while keeping her safe from the villain who might want to take advantage of her weakened state.
One possibility could be that an akuma's power goes wrong and instead of attacking Chat Noir in a normal way, it turns him into a baby. Ladybug is then left to protect this baby Chat Noir while also trying to find a way to reverse the transformation. She might have to take him to Master Fu for help, all while hiding him from Hawk Moth who could potentially use this situation to his advantage.
One great fanfiction with this concept is 'Baby Dean's Adventures'. It has a really cute take on how the other characters deal with a baby Dean. They struggle at first but then it becomes a heartwarming story of protection and family.
Perhaps it could be a character from an alternate dimension. In some fanfiction scenarios, portals open between different dimensions, and Spiderman could have had an encounter with a female character from another world, which led to the baby.
One possible plot could be that the magic backfires on Voldemort during a powerful dark spell. As a baby, he has no memory of his previous self. The wizarding world is in chaos as they don't know what to do with this baby - voldemort. Some might want to kill him immediately, while others think he could be redeemed. Harry Potter, having defeated him once, feels a strange sense of responsibility and wants to make sure he doesn't grow up to be evil again.
Well, one possible plot could be that his friends have to take care of him. They might struggle at first, not knowing how to handle a baby Izuku. For example, Bakugo could be really frustrated but gradually learn to be gentle. Ochaco might be really good at soothing baby Izuku when he cries.
The plot could be that Deku is doing some sort of experiment in the school lab when something goes wrong. A chemical reaction or a malfunctioning device causes him to turn into a baby. All Might hears about it and comes to visit. He feels a sense of responsibility as Deku is his successor. The other students are both amused and worried. They start to learn about taking care of a baby, like feeding, changing diapers, and making sure he sleeps well. Meanwhile, the teachers are researching how to reverse the transformation, and they suspect that it might be related to some ancient quirk theory that they need to dig deeper into.