How do you react to crazy parents in public?

I suppose your could call the police and tell them that she was threatening you. If you got her license number, they could identify her and find out if she has been reported for this type of thing before. But unless she has a record, I very much doubt that the police would do anything about it. She probably got the point from your staring and was just trying to save face by yelling at you. I would probably just let it go.