Seconding Ender's Game, White's The Once and Future King and Philip Pullman. I remember reading Douglas Hill's Last Legionary quartet when I was around that age, straight-forward well-written sci-fi for young'uns. Also, for Terry Pratchett, start with his written-for-kids books, the Bromeliad Trilogy is a good one to get them to question perceptions of the world, his Johnny Maxwell books are centred around a 12/13 year old boy, which might help draw them in. Both series cover stuff like racism, dogmatic religion, war and other stuff in a subtle way (Pratchett calls it "Stealth Philosophy") , good for sparking discussions with your kids.
Other than that, let them loose in a library. Cheaper than a baby-sitter.