There are many things you could get them it depends on their ages, what size bags you want to make, and how much you want to spend?
I made bags for my boys (6,11) using plain tote bag (from the craft store) that I emblished with their names with iron on letters. Inside I put a disposable camera, personalized tee shirts (made at disney direct), a journal to write about the trip (also embelished Disney style), a lanyard with a starter pin, a disney wallet with disney dollars, their favorite snack (for the plane ride), and of course a stuffed Mickey.
Hope it helps, Melissa