Would you have said something to this woman?

I would have said something but I would have done it in a way that the mother would have responded the way I wanted her to which would be to stop it from happening. If someone were to yell or be rude to her she would just feel defensive and get mad. So instead I would probably lean in close to her to keep it private and nicely say "hun did you realize your daughter is kicking the baby, poor thing" in hopes she would respond by stopping the behavior. Most likely she would act like she had no idea and then do something about it but if she responded by saying "yea they always fight" or something like that, I would just say "oh that's so sad" and leave it at that.