When you get them remember to get one of their fat pens or bring a sharpie. Make sure that if you buy the fat pens from them make sure that they write good, bought one years ago and it was hard to write with. I like taking the sharpies, they always work and now that they come in the click and close style I will like them more. Some of the characters have a hard time writing with small pens with their costumes.
This is kinda off topic but I really like this idea. When you have the character sign the book make sure they are all on separate sheets and only 1 to a page, (ie. have them all sign on the right side of the book), then take a picture of the kids with the character and then put the picture on the other page, so that when they are looking through the book later on when you get home, they will remember who signature it was. I hope I didnt confuse anyone with my ramblings.