A classic book for young and old alike, something that I read and was deeply moved by as a young boy, is Madeline L'Engle's "A Wrinkle in Time." The genre is sci fi fantasy, but it truly is a moral fable with a familiar yet uniquely presented theme. I highly recommend it as a vintage must for all pre-teens.
Best wishes to you as you go in search for good reading for your pre-teens and/or other loved ones and friends. I think it's wonderful that you are taking a proactive stance in furthering positive literary experiences for today's youth.