Think of anything, it can either from age regression, or your main character having to wear them for whatever reason (such as being a baby).
Also have you tried a game or looked at it because of the idea of your character being age regressed, or being in diapers for whatever reason? I can say I have actually
Anyway here are some examples with me:
Henry Hatsworth and the Puzzling Adventure (DS)-good game here, like one of the best for the DS IMO
Zelda: Oracle of Ages (GBA)-good game here as well IMO
Baby Jo in going home (PC, other)-meh, interesting idea for a game at least.
Bio Miracle Bokutte Upa (NES)-meh still, but interesting.
Imagine Baby Party (Wii)-not proud of trying this one lol.