Now I know why I don't shop at Walmart very often.

- But that's the crux of the problem. - Not faulting you for your reaction but when you stooped down to her level she was able to follow you further with her dumb retorts and probably felt a 'REAL' conversation was going on.

I pity anyone put into the situation you were and I agree that calling those brats down was better than doing nothing but you put yourself at risk when getting involved with lunatics.

I would have more likely reacted by putting an exaggerated dumbfounded look on my face and then turned my disgusted look back & forth between the mother and her monsters. - But some people have no shame and they probably would have continued their bad behaviors with their mother's blessings. I'm sure she thought they were being 'little darlings'.
Sound like my wife. Apathy will bring the end to this life cycle. Maybe next time we'll get it right