I would highly recommend the Holocaust Museum. It will take some time, and if it is like it was when I was there it may not be suitable for pre-teens. Some of the displays were pretty graphic. I saw grown men visibly shaken, and some like my father who pre-JW had been in some pretty bloody combat situations began to cry.
Anyone who wants to so freely throw the term Nazi around for political opponents today should see what Nazis really were, IMO.