My father O”H once came to my daughter’s kindergarten class, as part of a generational program. He was there to speak about Torah Klafim that he rescued during the Holocaust. Neither of my parents was a survivor, my dad came here when he was about 12 and my mom was born here. So his perspective was of a soldier fighting the Nazis.
So how does one explain the Holocaust to 4 and five year old children? He started off by asking them if they knew who Pharaoh was. He asked them to tell him about what kinds of “mean things” Pharaoh did to Bnei Yisroel, and several children offered their version of “slavery” to him. Then he asked them if they knew who Haman and Antiochus were, and of course they all raised their hands, and so forth.
THEN, he said that not so very long ago a man named Hitler was just like Pharaoh, Haman, and Antiochus, and he tried to be very mean to the Yidden. He didn’t like that they learned the Torah, he didn’t like that they did mitzvos. He made them live in tiny houses and didn’t let the children go out to play. He didn’t let them have enough food to eat. THIS was something the kids could understand and relate to without encumbering them with nightmarish thoughts.
He brought out the Klafim (which are encased in Lucite, and showed them how the bad men drew pictures all over the holy Sefer Torah, and then cut it up into smaller pieces. This too, they understood as being very, very wrong. He finished off by telling them that Hashem helped the Yidden and the soldiers to stop the bad people, just like the Chashmonaim. They got a little bit of modern Jewish history, appropriate to their age and understanding, without frightening them.