
I would consider (and have given to my nephews and nieces) short story collections. Short enough for them to work with but very rewarding. In particular Clarke, Asimov, Niven. For Heinlein I would stick to early Heinlein (see http://en.wikipedia.org/wiki/Heinlein_juveniles ), say : "Rocket Ship Galileo", "Farmer in the Sky"
I would also consider various short story collections. The great advantage is that if some of them don't resonate with one or more of your kids then next one will.
One new book worth looking at is Cory Doctorow's Little Brother http://craphound.com/littlebrother/about/
Also : with respect to some stories being "darker and more political". As long as they are also reading stories with a different messages / vibes then they will not even notice.
Chemist who falls in acid will be tripping for weeks.