Sure, you could get an autograph book for your kids that they’ll only rarely look at after your trip is over.
You could buy a larger autograph matte from a local store (target, bed bath and beyond, party stores, craft stores) and have the characters sign THAT! Then pop a picture in and you have a GREAT keepsake that you and your kids will treasure for years.
Make sure you get one large enough for many characters to sign. I think an 11×13 would be great. Just remember it will need to be carried on a clipboard (hard surface to sign on) and you probably want it to fit in your back pack for easy carrying. 🙂