If you're still wondering what present to buy for the bright young things in your life, Nature magazine has very helpfully reviewed stacks of children's books in yesterday's issue:
overview by Harriet Coles
101 ways to save the Earth -- and other books reviewed by Tom Standage and Ella (7 1/2)
Hawking for kids
... well, there's lots more, see for yourself. The web pages carry a label saying that you'd have to pay for full text access, but as far as I can see, the pages linked to above do give the full text for free.