Actually, I think 3rd and 4th grade is a perfect time for these types of “easy” moral analyses.

If the goal is to teach kids to think, then you want excerises building critical thinking. Even kindergarten is a good time to start critical thinking on hard topics “Why do you think the police shot those people with a hose?” I remember coming up when taking my 1st grader to the MLK museum.

If the goal is indoctrinate kids until it’s “safe” to understand why things are evil and wrong like slavery, then by all means wait until later. I’m not sure what the right age is.

I disagree that my reaction was exactly what the parent commenter was speaking about. If I said "you must be racist or pro-slavery if you even want to consider talking about this" then it would have been. But literally the first thing I said was that I understand the point, and that his heart was in the right place. The example works against his point - because it's so obviously a bad idea, isn't really worth defending and just confuses things further.