Doesn't have to be your kids - could be you yourself when you were a kid.
Here's one. I don't know if it was me or my cousin, we'll assume my cousin. So she's about three or four at a park or on a bus in Buffalo and she goes up to a black lady and points to her skin and says something like "Oh, you need to go home and take a bath, you're all dirty." I think the lady was understanding and explained that different people have different skin color.
Are You Embarrassed by Your Child's Behavior? 5 Ways to Cope
This one happened a couple days ago. My three and half year old asked one of our other kids' mom (who is very overweight) "Do you have a baby in there?"