That's very cool! And you pose a somewhat difficult question! As someone with another full-time job, it might be easy to think, "It's worth the publicity and a line in my CV." But even though they're a non-profit organization, they are going to SELL this product and there are many other cost-inputs. Moreover, it is not just inclusion in the book, it's the back cover. I imagine the person designing the front cover is getting paid.
So in short (beloved wife is calling me to lunch!) I think I'd ask, diplomatically, for some token payment. Explain your desire to be included and your intent to not impose an impossible financial burden, but the reasonable belief that art must be paid for, even if it is a token ($50?), or (maybe better!) a BARTER for something they may be able to provide for your schoolroom!
Best regards (and envious congratulations!)