I'd like to get my eight year-old a PC game for Christmas. Over the years, we've bought him (the previous) SimCity, Minecraft and he has played a few of the Nancy Drew Mysteries, plus a Carmen Sandiego. Because he watched some videos, we tried a Steam game (Gary's Mod), but it was too violent for our taste. (My wife thinks there's too much shooting in Star Trek Online, but though he really hasn't gotten the hang of it, we still let him play)

What I'd like is a (mostly) non-violent quest (maybe in space) or a building game (in space).

I know it's last minute, but does anyone have a suggestion?
Neither the wife or I play games, so we're at a loss.