It all really depends on the ages of the kids. But here are a few you might not automatically think of.....
Each day bring a change of clothes in a ziploc freezer bag. Once we broke down ona ride twice and our tot had an accident......New clotches will run you abut $60 for a shirt shorts and underwear.
Cheap rain ponchos and a rain cover for the stroller.
fruit snacks and juice bags.
Water ($3/bottle in the parks)