Obviously, Itachi as a baby would be a main character. Sasuke would also be a key character as Itachi's brother. Their relationship would be very different now that Itachi is a baby. Naruto could also be involved, perhaps being curious about this strange transformation and wanting to help out in some way.
The main characters would be Itachi as a baby, for sure. Then, the people in his village, like the elders or other ninja who knew him before. They would be trying to figure out what to do with this baby Itachi. Additionally, any friends he had made during his time as a normal ninja could be main characters. They might be worried about whether he'll ever turn back to normal and how to keep him safe in the meantime.
England could be one of the main caretakers. Despite their past differences, he would feel a sense of responsibility. Another could be Canada. Since they are related, Canada would surely step in to take care of his baby brother.
Obviously, Ash and Erica would be the main characters. Ash is the one who undergoes the transformation, so he's central to the story. Erica is the one who causes the change, so she also plays a crucial role.
Obviously, Ash would be one of the main characters, since he's the one turning into Victini. Pikachu would also be important as Ash's most loyal Pokémon. And Victini itself could be a key character, either as a part of Ash now or as an entity that helps Ash understand his new form.
Obviously, Harry Potter himself would be the central character as he is the one undergoing the transformation into a born vampire. His friends like Hermione and Ron would also be important characters. They would likely play a role in either helping Harry adjust or being conflicted about his new state.
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.
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.
Obviously, Jaune would be one of the main characters as the title implies he has a baby. The baby itself could also be considered a main character in a sense, as the whole story might revolve around it.
Pain could also be a major character. His ideology and the Akatsuki's goals under his leadership might be affected by the presence of baby Naruto. He might see Naruto as a potential threat to his plans or as an opportunity to change the Akatsuki's path. Also, Konan might be involved as she often has a more empathetic side compared to some of the other Akatsuki members and could play a role in taking care of the baby.
The plot could be centered around the chaos that ensues when Itachi turns into a baby. His enemies might see this as an opportunity to attack, thinking he's vulnerable. However, his allies would protect him. And as they take care of him, they might discover new things about Itachi's past or even his true nature that was hidden before. There could be many comical scenes like when they try to diaper him or feed him baby food and he makes a fuss.