Though this is a difficult subject for children to understand, there are many resources available to teachers and parents looking to start discussions with older children. It's imperative that everyone, young and old, know about this dark time in history and work together to educate others so that we never forget.
Selected Holocaust books from Kar-Ben:
AJL's Podcast "Teaching the Holocaust Through Picture Books"
AJL's Podcast "Not Just the Facts: Illustrated Non-Fiction for Youth" (fast-forward to 34:30 to hear Chris Nicola speak about Priest's Grotto)