Sci-fi for pre-teens by one who read them... (Score 1)

i discovered "the white mountains" by John Christopher ( pen name for Samuel Youd )when i was about 11 and was thouroughly hooked. I read all of his stuff i could get my hands on. Really great for kids and not too dark. Try the entire Narnia series as well. And, Ursula Le Guin's Earthsea books are excellent. The Great Brain is a series of children's books by American author John Dennis Fitzgerald, also one of my favorite reads but a bit off of sci-fi. I got more, but that should start and keep them very happy if they are readers. Happy Trails.