Now that DS (15) is bigger than me, he carries his own water bottle (on a strap), his phone and wears a light jacket or long-sleeved shirt in the mornings. I carry a Vera Bradley Disney print backpack -in the backpack I usually have a small wallet/coin purse which carries some cash, credit cards, park passes and one traveler's check; also hand sanitizer, sunscreen, lip balm, sun glasses, phone and/or camera and folding brush. I also usually wear a light layer in the morning. I carry my own water bottle on a strap as well. We usually go back to our hotel in the mid afternoon. In the evenings, I usually bring the same stuff, maybe leave out the water bottle in favor of a bottled soda which I toss when I'm done.
We don't bring extra clothes anymore. If we get dirty, we stay dirty until we return to the hotel.
We try to plan the days we will ride Splash Mountain or Grizzly River Rapids and bring either swim clothes or rain ponchos for just those days. Then I might bring a little bit bigger backpack but not a full sized one.
HTH!