I will never forgive Brooks for Sword of Shannara, which is simply a blatant ripoff. Maybe his other Shannara books aren't, but I don't find Brooks a good enough writer to look at them.

Clarke is generally fine for a preadolescent reader. 2001, Childhood's End, Rendezvous with Rama, and Against the Fall of Night are all good reading. The stuff cowritten with other authors tends to be terrible, though.

In the fantasy department, LeGuin's Earthsea series is excellent; same goes for Tolkien's works.

Some of LeGuin's Ekumen stories are appropriate for children; others... well... aren't. Read Four Ways to Forgiveness and you'll know what I'm talking about. (In case you ask, no, The Left Hand of Darkness isn't anything like that. It's barely erotic at all.)

Heinlein's Farmer in the Sky is good.