Robert Sabuda makes some of the most visually interested childrens books available today. I always love getting a new pop-up book from him. He has such talent. There isn't much variety in what's available on his website, but there is a lot of what he's good known for: pop ups!
First you can check out the Pop-Up Gallery to see lots and lots of pop-ups! The International Gallery contains examples of pop-up art from all over the world, including some from children. You can find tips for making pop-up books and awesome simple pop-ups that you can make!
Go make a cool pop-up with your kids!