I see...adults may YELL at their children as if they were MAD...but that is?

It's not okay for adults to yell at children. The best thing for you, as a bystander, to do is to intervene. Smile at the parent and child, offer your sympathy for their frustration/situation, ask if there's anything you can do to help make it easier for them. If you can't bring yourself to do that, make it clear to them non-verbally that you don't approve.