It could be an adult who had a very difficult childhood and is given a chance to go back and rewrite it. For example, a character who grew up in poverty and now gets to experience a better childhood.
Perhaps a character who has just gone through a major life change or trauma. Going back to being a kid could be a form of escapism for them. They would be the main character, trying to make sense of this new (old) world and figure out how to deal with their present - day problems through the lens of their childhood.