For the hotel: the kids get one or two bedtime storybooks and a "comfort item" each (e.g., favorite stuffed animal or blanket). We also bring their homework if they're missing school that week (with the goal that it gets done on the trip down, so they don't have to do it during the vacation). We spend so little time in the hotel room (just naps, freshening up, and sleeping) that it's not practical to bring more than that.
For the plane (we don't drive), they each get a care package with: (1) new sunglasses, chapstick and/or sunscreen, as needed; (2) new autograph books and pens, if needed; (3) snacks for the road (e.g., fruit gummies, Goldfish crackers, baby carrots); (4) a new sticker/color/activity book -- a lot of websites have Disney-themed free printable activity sheets and coloring sheets if you want to DIY; (5) colored pencils or washable markers; and (6) a simple travel game or card game.
For a road trip, I'd also add: little lap desks, neck pillows, a lightweight lap blanket, bottled water, a collection of fun singalong CDs, and electronics (e.g., Nintendo D.S.). If you have a portable DVD player, pack some Disney movies and/or the planning DVD.