You say your kids have read all the Harry Potter books and they're desperate for more? Why not try The Tales of Beedle the Bard? Five fairy tales from the wizarding world (including The Tale of the Three Brothers,' which played such an important role in the last Potter adventure) with commentary by Albus Dumbledore to put the stories into historical and cultural context. A must for anyone wanting to know more about Ms. Rowling's wonderful, magical world. Also check out Quidditch Through the Ages and Fantastic Beasts and Where To Find Them (soon to be a major motion picture)!
Translated by Hermione Granger from ancient runes and featuring commentary by Albus Dumbledore, a collection of imaginative fairy tales set in the fantasy world of Harry Potter includes "The Wizard and the Hopping Pot," "The Fountain of Fair Fortune," "The Warlock's Hairy Heart," "Babbitty Rabitty and her Cackling Stump," and "The Tale of the Three Brothers."