Depending on how much time you have, you could read the short
chapter book called
The Wave by Todd Strasser. I just finished reading that orally to my kids (5th grade) to go along with our unit on forms of government: monarchies, dictatorships, and democracies. It's based on a true story that took place in a high school history class where a teacher was trying to get his kids to see how easily it could happen that people would begin to follow a leader so blindly. There are a couple of "damns" in the book, but I just read those are "darns."

I also skipped over a line where it said along with the Jews, homosexuals and the feeble-minded were also considered inferior. Maybe I shouldn't have, but I did. (This was made into a movie in 1981.)
Now, for a very short hook, you could read the Dr. Seuss picture book called
Yertle the Turtle. Of course, Dr. Seuss is much more juvenile, but the storyline is about a turtle king who believes himself to be all powerful. They will probably get a kick out of hearing the picture book. The older kids don't get much of those, I imagine.

You could make it higher level by just reading it aloud (Use a document camera to display the pics, if you can.) then having the kids write out what they think was the author's message. That could start a discussion.
One other thing I'm thinking you could do with your kids would be to display some photos copied from websites about the Holocaust. Using photos alone could make a big impact and invite some deep discussions.