We don't do a large backpack because no one wants to carry it, so everyone in our family gets their own fanny pack.
Each child brings what snacks they want & their own disposable camera along with a hand held video game for waiting in long lines and a water bottle.
I carry the tickets, money, license for ID, one major credit card, everyone's Disney $'s, a few band-aids, facial sunscreen, camera, chapstick & gum.
Our rule is if you want to bring it you carry it. If anyone takes a jacket or sweatshirt it gets tied around their waist when not being used. We have done this since my youngest 2 (twins) were 5 & didn't use a stoller any more. It saves on arguments about whose carrying what & also saves on things getting lost or left behind because everything is always attached to the body somehow.
If we get souvenirs they get sent back to the room unless we are on the way out of the park when we purchase them.