I think that autograph books are a great idea! Before you go, however, it might be a good idea to ask your kids which characters they'd really, really like to see and then make sure you have some idea as to where the characters usually appear (allearsnet.com has some really handy lists). Also, make sure to ask cast members if, when, and where a character is scheduled to appear that day - they are always happy to help!
I hope that your family has a wonderful, magical trip!