Also, Harry Potter... though in that case, it was on purpose. But back when I first read them, I didn't think anything of a bunch of preteens fighting three headed dogs and dark lords and dealing with attempted mass murder. I mean, I always thought the books got darker in the third book, but when I think about it... yikes.

I feel the same way about Narnia. I actually haven't re-read the books as an adult, but I took my kids to see the movies and seeing those young kids stabbing ravenous yet sentient creatures with swords, and then getting stabbed themselves was somewhat traumatic for me--more so than for my kids, actually.