Our last trip was our first with a child (DS was almost 2 at the time). That really upped what we had to bring into the parks. For him: A change of clothes, diapers, swim diaper, wipes, kids sunblock, sippy cup, first aid kit (he is still new to walking and falls a lot), small snacks, small toy/book, hand snaitizer, and of course his stroller. This is on top of our stuff: ponchos, sun block, camera, Pal Mickey, chapstick.
I learned very early to eliminate ANYTHING I did not absolutely need. I usually carry a wallet, but at WDW, I took only my license, room card, and one credit/debit card wrapped in an elastic. No cash means no change (which can get very heavy). I didn't take the whole box of bandaids, just 2-3.