Passports- In world showcase at Epcot as you travel around the different countries, kids can get a CM to sign an Epcot passport book. The Epcot map will post location of these stations. They will give a stamp for the country as well as write child's name in the language of the country, etc. These passports can be purchased there- but they are expensive and plain. Several of the wonderful designers here on the boards make their own varieties of the Epcot passport that can be printed out at home and bound together.
ID cards- just something simple in case your child gets lost.
Coupon books- Many designers have created "coupons" that you give to your child as a means of promising them special treats at the world. i.e. a mickey ice cream bar, mickey ears, a popcorn bucket. Simply a way for the kids to get more excited about the trip and the goodies to come. You can print these out from the designers and choose as many or as few as you are willing to purchase for your child. There are numerous options and designs out there.
Hope this helps.
Good Luck!!