Agree with @TORquoisecoast.
I used to call people out on bad behaviour. But now I bite my tongue. So many people are out of their head on ice and they have no boundaries.
I value my own safety, and that of my children, over putting someone in their place.
You did the right thing and handled yourself with class. Rest assured this would have annoyed her way more than anything else you could have done.
Imagine if you said something and she did something which either deliberately or inadvertently hurt your DS? Not worth the risk.
You put your child's needs first. That's the sign of a good mum.
The only time I probably wouldn't have controlled myself was I remember hubber saying a lady spat on her daughter. Not sure I cold have controlled my temper then....