The original post and replies scream troll post, but I will bite.
She was wrong to comment. If she is actually worried about the child's safety, she needs to call the police. All she did was make the mom more angry and therefore endangered the child even more. When someone is driving aggressively and you honk or gesture at them, do they tend to realize their faults and slow down? Or do they double down and try to make a point?
There is an anecdotal reason I am responding to this post as well: my wife was in the grocery store about 10 years ago with my daughter (then 3 years old), and she was in the same aisle as another woman who was cursing into her mobile phone. Just terrible language, and very loud. My wife asked her to please not curse around our daughter. The lady went ballistic and almost attacked my wife, had a store manager not stepped in to help. They had to call the police, although my wife didn't press any charges.
So if you find yourself in this position, do not engage the person. If you think their child is in danger, make a phone call to the authorities and be prepared to back up your claim.
Or, because it is 2020, you could just film it and post it to Twitter because that is what seems to be popular these days.