Tell Them One Thousand-and-One Times and Then Change Your Tactic
If you were watching that parent scream at his child, you might suggest he try another tactic rather than hostility and demands. You might suggest that another approach to his child might bear a more fruitful result. You might suggest that his frustration and rage, while perhaps completely justified, is not achieving the result he wants. And, sure, he might tell you to stick it up your ass but at least you tried.
...that the cruel joke is that prisons and cliffs share the same architecture: an edge you’re afraid to cross. The only way to learn which one you’re standing on is to step forward and trust gravity to reveal the truth.