I'm a crossbody gal, and this one is my hands-down favorite:
http://www.ebags.com/product/overland-equipment/bayliss-crossbody/280027?productid=10321632 . The ballistic nylon body and seatbelt type straps are easy to clean, and this particular bag converts to a waist-pack with 3 prongs so it can be worn on nearly every ride at Universal. It's also discontinued, though it's available several places in less popular colors. It holds an amazing amount of stuff: travel size wipes, small baggie of band-aids, travel size advil and tylenol tubes, small triple anti-biotic ointment, 2 travel size sunscreens (refilled each night), lip balm, gum/mints, smartphone, 1-2 disposable ponchos, cards & cash, glasses cleaner & cloth, folding scissors, hair ties, even a very small hairbrush, plus a few other odds and ends. My husband and boys carry their own emergency ponchos in cargo pockets, and we have water bottle holders that clip to purse or belt (or into the other cargo pocket). That takes care of everything we need for a day in the park, and has since our boys were about 7 & 9 (they're teens now).
My other fave is another ballistic nylon bag with seatbelt straps, but it was discontinued even longer ago. It's a little smaller, so the boys have to carry more, but so much easier to deal with than a backpack. We've tried that, too, and they just get too hot unless we are using one of the good hiking daypacks. But hubby does not like to take those into the parks, and they are harder to fit into lockers if we decide not to carry one all day.