In addition to disney dollars, how about...
Travel kits
for the kids -- fill a fanny pack or kid-sized back pack with a sports bottle, some snacks, disposable camera (my DD used one on her first trip when she was 3 and took some great "kid-view" pictures of disney), pipecleaners (a great quiet plane toy), autograph book with fat pen or marker, picture books, crayons, mini-playdoh, a stuffed animal or cuddle toy for the plane, etc..
for the adults -- fill a backpack or fanny pack with travel items like wipes, antibacterial lotion, travel-sized kleenex, band-aids or travel first aid kits, gum, moleskin, dental floss, calling card, etc.
Are they driving from the airport to WDW? Maybe a
AAA map of the area so they can find their way around. Certificate to a favorite restaurant for when they return and experience the unpacking-washing-disney let down day that follows your return.