For this project and a first printing, I'm doing the books in paperback for a couple of reasons. The cost to produce a hard cover book is considerably more expensive than a paperback. That being said, I'm using one of the best printers possible to go to print with the books, so the quality should be awesome.
It keeps the price of the books affordable for everyone. As I continue to build the series, the last thing I want is for people, especially in this economy, being unable to purchase the next book in the series because it is too expensive. Your wallet wins - and your kids get to keep enjoying the book series!