I absolutely adore kids' sci-fi and fantasy, like the classic A Wrinkle in Time. I loved (and hated) the Animorphs series by KA Applegate, and her other series Everworld is good too. Both of those absolutely must be read in order, though. Then there are all the alleged "juvenile" novels by Heinlein. Good stuff there, particularly Tunnel in the Sky, which got a mention on SG-1. And anyone who hasn't read the Rats of NIHM is really missing out.