Traveling with children

There was a post on FB that had a child on a plane flight who was yelling and screaming and a passenger apparently yelled back at the child. How do you deal with someone else's child who is being loud doesn't matter the age. Would you help the mother or just sit and deal? Also for the mothers who have gone through having a child that cries and screams let's say... Two and up how do you call them down? And if u can't how do you handle the situation?