The reason I didn't say anything in the moment was because I didn't want to lose my temper. I thought it was best to go home and write a text message where I could be in better control. Also, I was in her house, and a conflict there wouldn't have reflected well on me. The worst case would have been that she called the police and I would have ended up in jail.
I do agree with you completely though, you can't stand around and let your kids be disrespected.
I like the story you shared about the man pushing his car. Sometimes just offering help takes a huge amount of pressure off somebody. Thanks for the thoughtful comment.