We use snap diapers & one piece pjs. Baby Gap online (not in stores) has one piece summer zipper pj's in up to a size 5T.
We've been using these since my little guy wears a size 5 and I've yet to find anywhere else that sells one piece summer pjs.
Also try putting things on backwards. Backwards diapers & backwards one piece pj's if they can undo the zippers or snaps.
-mom to 3 big girls (9, 12 & 18 ) & 1 little guy (26 months)
Mom to my girls ('94), ('99) & ('02)
And my little guy (12/09)