We bought a collapsible backpack from the camping section at
Walmart. It folds into itself & has a draw cord and was packaged at about the size of a fist. When it unfolds, it's about a rectangle a little more than a foot tall & a little less than a foot wide with one outside pocket on it.
In it, we carried an extra outfit for DS5 in a ziploc bag, small container of sunblock, ziploc bag of wipes, 2 bottles of water some days, ids & extra cash in a ziploc bag, hand sanitizer, bandaids, antibacterial ointment (lots of skinned knees), a flashlight, 2 cameras, a waterproof camping bag, a tiny bottle of bug spray, extra ziploc bags, and glow in the dark bracelets for the kids. It was a also good place to store the maps we picked up along with extra napkins & small purchases. Get a bag with very few zippered compartments because they check them all.
DD10 carried a fanny pack with a tiny bottle of water and her autograph book & sharpie, some of her own spending money, and the quarter tube she used for pressed pennies. Bag checkers checked her bag, too. When she got tired of carrying it, it went into our backpack.
We took that bag on every ride. When we went on Splash & Kali, we put our cameras & cell phones in ziploc bags, put them in our backpack, and put our backpack down in the waterproof camping bag. Everything stayed dry!