I have found that no matter what, all items migrate to mom, especially if you dont want them lost. Consider that you will spend all day reminding everyone after every ride, show, parade, bathroom break etc to look after their bag, that it will just be easier for you to carry it and therefore defeat the purpose of everyone carrying their own bags.
I am with the camp that recommends to bring as little as possible...consider a caribiner clip/holder for a bottle of water and small sunblock, chapstick and a camera in a waterproof/ziplock bag that can be tucked in a few pockets.