The other day I left work early and got on a different bus than I usually do. It was moderately crowded and in the front section I was in, there were two other white people—a mother and her 8-ish year old son—sitting right in front of me.
It was generally quiet except for three chatting black women seated in front of the mother and son.
After one of the women says something using the word ain't, the little boy says (loudly) to his mom:
"Mommy, why does she say ain't? That's not a real word! Why do black people always say everything wrong?"
The women glared.
The mom, to her credit, looked mortified and shushed him.
I find African American Vernacular English a really interesting topic—I think it exists not as a 'lesser' English, as it seems to often be viewed, but as an entirely unique language with its own complex structure and set of rules. Here's the wikipedia which does a pretty good job of documenting that.
What would you have done as the mom in that situation? Say that it's not wrong "but you still can't speak that way"? Just not address the question?