I mean it depends on the child, when I was in sixth grade I could read pretty much every book I can read now and did and I'm glad I read it as a child and had no problem understanding it, and many other children also have no problem understanding it ever since it came out. if you didn't know enough about the history of racial oppression it's your educational system's fault, especially if you actually live in the US but often in other countries we learn more about that than the locals in some places