I've been involved in an interesting conversation about reluctant readers, started in the comments on Jen Robinson's PBS Parents Post, and continued on her blog. One thing all the commentators seemed to agree on was that kids need to find their own level, read the books that speak to them rather than what seems to be popular. And, of course, I agree.
But. I also think reading above one's level is actually a fine thing--if one is the kind of kid who stretches rather than shrinks when confronted with new things. I was that kind of kid, and I read War and Peace when I was twelve, after seeing the Masterpiece Theatre production. I can't say I understood it, but it gave me great pleasure, nonetheless, to cart it around and do my best.
Now A.O. Scott chimes in with a similar, but perhaps even more revolutionary, suggestion--that you let your kids watch movies "above their level." Again, speaking from personal experience, I'm with him: I know I watched things that I wasn't "ready" for, emotionally or intellectually, but that I nonetheless stretched and grew as a result. Of course this all has to do with knowing your own children--Nick found both Harry Potter and The Princess Bride "too scary" the first time he tried to watch them, so we let him stop and he came to them on his own time. But Mariah started watching PG and PG-13 movies long before she was 13, if we thought they were right for her, and Nick has certainly seen his share as well. Perhaps of R movies, too, though it's hard to remember.
Books don't come with ratings or warning labels (usually), of course, but I think the issue is analogous. Share what you like, know your child, and enjoy.