1. The Holocaust, the events going on during and before the war. Camps, Ghettos, etc.
2. When the war actually started! Many of my young friends think it started in 1944 with D-Day . But it started in 1939.
3. Young people should also learn about the Atomic bomb and the devastating results it caused for Hiroshima and Nagasaki.

Those are the 3 most important ones off the top of my head...