Buy at least one disposable camera for each child on the trip. These are very inexpensive and will keep your child entertained for a long time. Tell each child that they should take pictures of things on the trip that they find interesting. Mom and dad should have one too.
When you get back home and develop the film, each person should make a scrapbook and tell the family why they took each picture.
You may find that traveling through a child’s eyes can be even more rewarding than the trip itself.