I love this idea. Start a savings shadow box with a picture of what they're saving for. This would also be cool for the kids, to teach them to save for something. I think I will frame a vacation spot I want to save for. Good idea not just for kids.
You can order a box of books from this site for $12.99. They will range from toddler to young adult books. Each box contains approximately 150 books (give or take). You can also order more age/grade specific books, but it costs a bit more. There is now an advisory on their site that due to high demand, boxes may not ship for up to 5 weeks, but for $12.99, thats a small price to pay :)