Obviously, Sonic himself would be a main character, even in his baby form. His friends like Tails, Knuckles, and Amy would also be main characters. Tails could be the one trying to figure out how to turn Sonic back. Knuckles could be the protector, and Amy could be the caretaker.
Obviously, Voldemort as a baby would be one of the main characters. Harry Potter could also be a main character as he has a long - standing enmity with Voldemort. He might be involved in deciding the fate of the baby - voldemort. Dumbledore would surely play a major role. His wisdom would be needed to deal with this strange situation. And Hermione, with her vast knowledge of magic, could be involved in trying to find a way to either reverse the transformation or ensure that the baby Voldemort grows up to be good.
Obviously, Ash would be a main character even in his baby form. Pikachu is bound to be a key character as well, being Ash's closest Pokémon partner. His other Pokémon like Bulbasaur, Charmander, and Squirtle could also play important roles in protecting and taking care of baby Ash.
Obviously, Shadow himself would be a main character even in his baby form. Sonic would also be a key character as he is often the hero in these Sonic - related stories. He would likely be involved in trying to figure out how to turn Shadow back. Tails could be another main character as his intelligence would be useful in finding a solution to Shadow's transformation. He might study the cause of it and try to come up with a remedy.
Dr. Eggman would be an obvious choice. Since Sonic is now in a more vulnerable human form, Eggman could see this as the perfect opportunity to finally defeat Sonic and take over the world. He might come up with new devious plans, like creating robots specifically designed to target a human Sonic.
The main characters are, of course, baby Sonic and Amy. Baby Sonic is a younger version of Sonic, with all his speed and charm but also with a more innocent and naive side. Amy is known for her strong will and her love for Sonic, and in the fanfiction, she often plays a protective or guiding role for baby Sonic.
Mario would definitely be a main character. Since Luigi is his brother, he would be the one most concerned about Luigi's transformation and would do everything he could to help him. Princess Peach could also be a main character. She might offer her castle as a safe place for baby Luigi and use her resources to find a solution. Toad could be there too, helping Mario with taking care of Luigi and providing useful information about the kingdom that could lead to reversing the spell.
Perhaps a mysterious stranger. This stranger could have some connection to the reason why Edward turned into a baby. And then there's Edward's pet dog. The dog might be confused at first but then becomes very protective of baby Edward, adding an interesting dynamic to the story.
Her best friend could be a main character. They might be the one who is most affected by her transformation and has to take on a lot of responsibility in taking care of her.
Erza could also be a main character. Given her strong and motherly side, she would probably take on the role of taking good care of baby Lucy. She might be strict about following a routine for baby Lucy's care. Happy, the flying cat, could be another important character. He could use his unique abilities to search for clues on how to turn Lucy back to normal.
His guildmates would surely be main characters. For example, Natsu could be there. Natsu might be really confused at first but then would be super protective of baby Laxus. Another could be Lucy. She might be the one who is more practical in taking care of the baby, like making sure he has proper clothes and a comfortable place to sleep.