His friends would probably take care of him. Midoriya would be very eager to help, using his kind-hearted nature. He would make sure Todoroki is fed and comfortable.
Well, in this kind of fanfiction, probably his friends would be super shocked at first. They might struggle to take care of him as he's now a baby. His powers could also be affected in some strange ways, like they might be uncontrollable or show up in unexpected forms since he's in a baby's body.
France would be a likely candidate. France has always had a sort of motherly - like affection for Canada. He would probably be very eager to take care of baby Canada, dressing him up in cute clothes and feeding him French cuisine, like crepes.
Japan could also be one to take care of baby Italy. Japan is known for being meticulous and organized. He might be able to handle the practical aspects of taking care of a baby like preparing proper baby food and creating a clean and safe environment for baby Italy.
Well, in a Todoroki turns into a girl fanfiction, perhaps there's a villain who has a transformation quirk and uses it on Todoroki as a form of psychological warfare. Todoroki, now a girl, has to find a way to reverse the effect. In the process, he/she realizes that society treats female heroes differently and has to fight against those prejudices too. Maybe he/she even uses this new form to her advantage in some battles, by using the element of surprise.
His relationship with Midoriya might change. Midoriya could become more protective of Todoroki as a girl, seeing her as more vulnerable. With Bakugo, there could be a different dynamic. Bakugo might initially tease Todoroki more, but as he sees Todoroki still being strong in her new form, he might develop a new respect for her.
Family is a common theme. Since Shoto has a complex family situation, fanfictions often explore how his family members interact with him as a baby. For example, how his father might view a baby Shoto with his powerful quirk.
In the fanfiction, their relationship could start with a bit of competition. Both are ambitious to be heroes. However, they soon realize that they can achieve more by working together. As kids, they might compete in small challenges like who can run faster or jump higher using their quirks (or in Izuku's case, his physical training). This healthy competition could fuel their growth and transform into a deep - rooted friendship that withstands all the difficulties they face later in their hero careers.
One popular baby Todoroki fanfic could be 'Baby Todoroki's Adventure'. It might focus on his early days and how his unique quirk starts to show in a cute and innocent way. Another could be 'The Little Todoroki' which could explore his relationship with his family members at a very young age.
His friends like Sakura and Sasuke might step in to take care of him. Sakura, with her nurturing side, could handle feeding and cleaning him. Sasuke, though he might seem reluctant at first, would probably also protect the baby Naruto from any threats.
Lots of fans might write this type of fanfiction. Some might be amateur writers who just have a passion for the 'My Hero Academia' series where Todoroki is from. They could be using this as a way to express their thoughts on abuse and how it could play out in the context of the story. It's really hard to say exactly who writes it though, as it could be anyone from around the world who loves the character.