I think it is definitely easier to pick out fantasy books for kids. Here are a few that would probably be age appropriate and which are excellent books:
The Hobbit, J.R.R. Tolkien
Prydain Chronicles (first one is The Book of Three), Lloyd Alexander
Earthsea (A Wizard of Earthsea), Ursula K. LeGuin
Narnia (The Lion, the Witch, and the Wardrobe), C.S. Lewis
The Dark is Rising, Susan Cooper
For Sci-Fi, I think it is a bit harder to find good books for kids - most of the sci-fi geared towards children I utterly loathed growing up. But a few you might try:
A Wrinkle In Time, Madeleine L'Engle
Thrawn Trilogy, Timothy Zahn (Star Wars books, so assuming familiarity with the original movies - basically fantasy in space)
Robotech, Jack McKinney (not the greatest literature, but I loved these books at that age; might be hard to find today, though)
Some others, not too sure whether they are age-appropriate but books I enjoyed around that age (plus or minus a couple years):
Gateway, Frederick Pohl
Ender's Game, Orson Scott Card