Anne McCaffery has some good ones, but they are generally dragon & space oriented. Pretty good reads, and there's quite a few in the series.
Orson Scott Card has Ender's Game (and several more in that series). These are definately classic.
A Wrinkle in Time by Madeline L'Engle, and the follow-ups are all very well written, though some of the deeper themes might be a bit above your kids depending on how sharp they are.
CS Lewis' Space Trilogy is excellent, though it gets pretty violent, and might be a bit advanced for pre-teens.
Terry Pratchett's books are funny, but they tend to spoof the politics and happenings of the US and the UK, so your kids might not grasp all the jokes. Much better would be Douglas Adams' Hitchhiker's Guide to the Galaxy and the (increasingly misnamed) Hitchhiker's Trilogy (there are five books there).
If you like, you might even start them on JRR Tolkien, which is more fantasy than sci-fi, but definitely a classic. You also have the advantage of the movies once they're done with the books. (Books are better though.)
Those are my picks, and that should be enough reading for at least this summer, if not longer. You can also walk into your local Borders and ask someone. There's tons of great kids books in Sci-Fi...