Can we all think on the fact for a second that this probably won't make the clerk think twice -or- suddenly make them not racist, since the first thing you think of as a clerk when confronted with yelling is basically to, for lack of a better term, not engage with them; "tune them out"? I know anytime a customer yells at me (though typically for stuff I had freaking NOTHING to do with, holy *) I typically try to move them to the back of my mind so as to not blow up myself. Though I do have some issues with anger management, so I don't know. Maybe that's just me.
Not saying the Mother did anything wrong, not saying the clerk is in the right. I just don't think this was likely to teach a lesson like many of you seem to be saying.
Edit: right, and let me cover myself here before any false accusations and say that pretty much all my angry customers have been White.