I've read a lot of posts about kids losing interest in a book full of autographs over the years - but this way you would have the photos with them so they would be enjoyed for much longer.
Buy a normal 6x4 photo album - there's loads of disney ones around. Then take a stack of 6x4 cardstock (blank or decorated) on a mini clipboard to the parks for the characters to sign.
When you get home insert the cardstock with the signature in the photo album with the related photo on the facing page.
Have a great trip!