Lemme try fetch some ideas. got a bunch but they're not really developed xD

1) A boy goes to spend a week on some friends house, and gotta share the room with the toddler, that's from a bgiger species and wears the same size of clothes. He ends up trying some of his clothes and get caught, and ends getting the toddler week too.

2)Magic plushie dimension. a boy goes there with magic, meet plushies, stay some time there, but the plushies get his wand and keep him there as a little one (diapers included). by the end he either accepts there, or the plushies use his wand to make him stay there.

3)A little girl from kindergarten gets a crush on an older student. She then uses her magic to regress him so they can share the same class

For the spooky season, 2-4 characters go trick or treating when they come across a cutesy looking house/spooky mansion, they ring the doorbell and another character opens it and tells them they have a 'big surprise inside', they fall for it and eventually get diapered and regressed by the owner.

A diaper that turns u into a goo monster

A diaper tf virus?

A diaper that makes makes the wearer weaker overtime.
In no time any muscular and fit person can become so weak and scrawny, they have to crawl.

A diaper that NEVER fills up. Every time the wearer uses it, the diaper inflates to hold more. =^w^=