Your a human who lives in Equestria who babysits. but you don't babysit foals between 0-2. You babysit a character who has been mentally regrssed into thinking there a baby or toddler. This would also mean the adult or school aged pony would be wearing a diaper and using it. Who would it be.

Maybe applejack or twilight

I would randomly pick the little fluffball Celestia

7442962
Probably the Super Duper Party Pony, Pinkie Pie.

7442962 A mentally regressed Fluttershy shouldn't be too much trouble.

7443216
it would be cute

Cmc after a potion misshap. Baby sitting while manesix go on a quest for ingredients to reverse it.

flash sentry and twilight sparkle

7443625
you have just inspired a story I could work on

7444376
Sweet, I'll read it when its done. :) Glad I can inspire things.

7444418
ok