I wouldn't choose that book for a six year old with the specific needs you mention. In particular, I'd wonder if you are ready to discuss the police figures being bad in some instances, people being not only cruel but inhuman to children simply because of their religion, and the overall sadness of the Holocaust.
I can say that reading another book from that period, albeit a nonfiction work rather than fiction, did lead to my dd14 wanting to have extensive discussions about those topics and to us reading Stanley Milgram together to help her work through how people could have let this happen. She was a bit older (8), so she was a little more into a place where everything wasn't black and white, but it wasn't just a conversation to have or book to read and then move on.
