Understanding racism is somewhat complex; teaching about racism is very complex
A kid is bound to ask (or, at least, wonder): Why would one group of people enslave another group of people?
The complexity arises from an attempt to deal with that question.
There is a kind of straightforward answer: People want Others to do the odious labor.
But then you have to deal with: Why is there so much in the way of odious labor?
I think only those who recognize the centrality and broad implications of that question can provide the full context for teaching about the issue.
(For example: The South African apartheid system avoided direct enslavement, but was only a step removed from it. Relatedly, what right-wing Israelis want now is to wind up with something similar and thus have the Palestinian Others available to do the shit work.)