We pack:
plastic throwaway ponchos
sunscreen
autograph supplies (usually a small booklet of bound index cards and some Sharpies)
band-aids
wallet (with ID, credit card, and AP/ DVC cards, cash)
small pack of Clorox wipes (new addition after our last two trips- tables can get nasty)
powder (DS11 chafes pretty badly and I haven't always escaped that fate either)
charger for phones, and cables
hats/ sunglasses or both (usually we are wearing these, so technically not in the backpack)
water bottles (everyone usually carries their own so technically also not in the backpack)
DH has an
Amazon basics backpack meant for carrying his DSLR, and the back is vented so he doesn't get hot/ sweaty because of the backpack. I carry my DSLR in a small cross-body bag by e-bags.
For DS2.5 I toss in some diapers and wipes, and a small tube of diaper cream (I realize that doesn't apply to your kiddos, but for those following along at home, this all fits in one backpack so even when we have the stroller we don't load it up with stuff).