Perhaps there are other kids around baby All Might's age. They could be his friends or rivals. They might have different powers or abilities, and their interactions with baby All Might would shape the story. For example, there could be a girl with super speed who is always challenging baby All Might to races, which helps him improve his own skills.
Well, obviously baby All Might is a main character. He's likely to be the center of the story, with his cute and innocent personality.
There could also be a mentor - like figure. Maybe an older hero who takes baby All Might under his wing and teaches him the ropes. This character would be important in guiding baby All Might's growth.
Sakura herself would obviously be a main character, just in her baby form. Her best friend could also be a main character. For example, if she has a friend like Tomoyo, Tomoyo would likely be very involved. She would be trying to figure out how to help Sakura and also taking care of her, making sure she has cute clothes even as a baby.
Obviously, Izuku and All Might are the main characters. Izuku is the one with the relationship to All Might described in the title, and All Might is the object of Izuku's admiration as in the 'little girl' - like admiration.
Obviously, All Might would be one of the main characters. He is the 'dad' figure in this fanfic. The girl could be an original character created just for the story. She could be a young girl with a unique quirk, like the ability to manipulate light, for example. Her name could be something like Lily.
Ginny herself would obviously be a main character, even as a baby. Then Harry, Ron, and Hermione are likely to be main characters as well. They are her friends and would be the ones most likely to try and solve the problem of her being turned into a baby. They would be involved in taking care of her and finding a way to reverse the transformation.
Obviously, Keith himself in his baby form would be a main character. His friends like Lance, Pidge, Hunk, and Shiro would also be main characters as they are the ones who would most likely be taking care of him or trying to find a cure. Maybe Allura and Coran could also be important characters, helping out with their knowledge of Altean technology or magic that might be related to Keith's transformation.
Often, Recovery Girl is paired with All Might in fanfics. Given her role as a healer in the My Hero Academia universe, and All Might's frequent need for medical attention, it's easy to see how a relationship could be built around that. Writers might explore how she cares for him not just physically but also emotionally, which makes for an interesting story.
Perhaps a plot where baby All Might befriends other baby superheroes - in - training. They could all have little adventures together, learning about their powers and how to be good heroes. For example, they might have a race to see who can fly the fastest or lift the heaviest object, with some comical mishaps along the way.
Well, clearly the main characters are baby Harry and baby Ginny. Harry, the famous Boy - Who - Lived, and Ginny, who will grow up to be an important part of Harry's life in the Harry Potter series. As babies, they are the focus, and the fanfic likely explores their early days and any special moments or relationships they might have at that young age.
Well, obviously the baby is a main character. It's the central focus of the story.
Kuroko as a baby is the central character. But other characters from the opposing teams could also be important in an unexpected way. Maybe a rival team's player discovers Kuroko's situation and has a change of heart, deciding to help rather than take advantage. And of course, Kuroko's schoolmates who are not on the basketball team might also get involved, adding a different perspective to the story as they are not as familiar with Kuroko's basketball skills but still care about him as a person.