There is a free to play Korean MMORPG Called Blade and Soul where the main villain named Jinsoyun, is defeated by the your character the main protagonist, instead of killing the main villain whom the protagonist swore to kill to avenge their martial arts school, decides to purify her soul, in doing so she is regressed back to a preteen and taken to a world where she would be free to start her life anew... while she is in this world (off screen) she continues to regress smaller and smaller and gets sent back to your world in the next chapter as a toddler/baby to be protected and re raised by your character since your character becomes the master of the school and is the only one who is able to protect her from the evil to come.
regression to preteen starts at 3:05https://www.youtube.com/watch?v=wUCKXwDO8ws
aftermath baby Jinsoyun https://www.youtube.com/watch?v=p39hA5AuQj0
Next one is Black Cat the anime series
The main protagonist named Train gets shot by a special nano machine infused bullet that regresses him into a child and he spends a whole episode trying to adjust to living as a child and regaining old memories he had as a child while the two female protagonist that are normally younger than him are excited that they get to babysit him for a day as his big sisters and re raise him to be well mannered.
regression starts at 20:56https://youtu.be/PdvaI-9ROGU
This is the episode that he spends as a childhttps://www.youtube.com/watch?v=qGOpP-6zkmk