I would, really, really try not to bring a back pack if you can help it. Fanny packs (belt bags) are the best! Hands free and you don't have to worry about putting it down and forgetting it. They also carry quite a bit more than you'd think. Everyone in your family can wear one and you can split up the load that way. Buy a water bottle strap in the park (or buy one at home and bring it with you) for the water and everything else from your wallet to your camera can fit in the fanny pack.
In one bag, I can fit my wallet, camera, extra film, room key card, a few individualy wraped Wet Wipes, a small Kleneex pack, sunglasses (unless I'm wearing them) and a mini-bottle of sun screen for mid-day reapplication! Others in your party can be assigned the snacks, water bottle(s) and all the other stuff you need. Kids get a kick out of being "in charge" of something. By having the smaller bags, it also helps you cut down on the unessesary stuff.
You can have souveneers bought in the parks sent to the front to pick up before you leave or sent to your Disney hotel. Great service!
Unless you have a baby leave the backpack in the room or at home!