Would you have said something?

My daughter (Maddie, she's 4) and I were recently at a winter carnival in our town. Well, we were walking around, and we saw a photo booth (the kind they have in malls where a machine takes your picture), and Maddie wanted to have her picture taken. Well, we were the only ones there except for another family with a little boy and girl who looked about 5 and 6. Anyway, the father was yelling at his kids. He was saying, "You get in that photo booth, and don't you dare make any faces. I want this picture to be nice for your grandmother." Well, the kids looked terrified. The little boy stood up, and the father grabbed him, and yelled, "Look you little @sshole, I've had just about enough of you. Do you want me to hit you?" At this point, both kids were crying, and the kids asked for their mom who was standing there. She just looked at them, and told them to listen to their father; to which the father replied, "See your mother isn't going to help you, so sit down and SHUT UP." I didn't know what to do. I had my daughter with me (she was crying by the time it was our turn. she asked me why the man was so mean). What would you have done?