When I was in fifth and sixth grades, I loved the Tripods trilogy, by John Christopher. "The White Mountains", "The City of Gold and Lead", and "The Pool of Fire" are beautiful, powerful novels for children, but I was surprised how moving I found them when i reread them a couple years ago. (It's safe to skip the prequel, "When the Tripods Came", which was written 20 years later. It's not bad, but not up to the standard the others set.)

They're not "hard" science fiction; the science is sketchy at best. But they have a great postapocalyptic setting and a delightfully creepy vibe.