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)!
A collection fairy tales for young wizards and witches, with each story followed by observations on Wizarding history, personal reminiscences, and information on the story's key elements by Hogwarts headmaster, Albus Dumbledore.