I had a good one today when I took my youngest two (ages 8 and 6) along with me to vote in a special election for County Executive. They had lots of questions:
"Are you voting for President again?"
"What's a county executive?"
"Why did the lady ask for your name and where we live?"
"How do you vote?"
So, we were able to discuss:
... and so on.
Now this isn't upper level civics, by any means, but it was a chance to introduce big concepts for little people to think about.
Of course, they also learned that voting is important, because they went along with me and saw that I thought it was a very important duty of citizenship.
Maybe that was the best thing of all for them to learn.