My dd13 read the diary of Anne Frank when she, too, was eight. For her, the hardest part was trying to understand how people could have let this happen. That led to us reading Stanley Milgram's Obedience to Authority. I think that the search for justice among gifted kids makes understanding the "how" of WWII difficult as well.
It does sound like your dd is more sensitive to the "what happened" piece than my dd was/is, though. I haven't asked dd not to read books specifically but I have given her a heads up as to what she'll be coming in to with specific books and discussed them with her. There have been some she's chosen not to read following our discussions. For instance, Speak was one she passed on when she was 10 after we discussed that the main storyline was about a date rape. I think that she's going to pass on the Girl with the Dragon Tattoo after she and I discussed that it includes bondage and some weird sexual stuff.
Maybe talking with her about what she's in for in reading the book and letting her decide if she thinks she wants to try handling it would work.
