Any books/series that could be understood, even slightly, by 6 year olds, but would still be of interest to a 10 year old and 12 year old? In the past I've read all the Roald Dahl books. Then we did part of Harry Potter until it got too confusing for the (then) 7 year old. We also did the cricket in times square series (yes, it is a series, but there is a reason people don't know the other books!)
Survival, backwoods, families surviving the elements type stories?
Fantasy stuff not so appealing.