In "Once upon a Time" by Nadine Gordimer, how could the parents have prevented the tragedy of the end of the story?
The tragedy at the end of the story, the death of the couple's son, is a culmination of their mindset and decisions throughout the story. At a minimum, after installing the Dragon's Teeth fence, since they had decided to do so, they should have built another wall inside of that wall to keep their son safe, or perhaps rig up an electric fence to keep him from crossing over to the dangerous wall. They may have warned him not to go near it, but that's not enough, as they realized when they put a wall around their swimming pool to protect their son.
Better yet would have been to not install the Dragon's Teeth at all. The wife had misgivings after it was installed, fearing for the cat. She should have trusted those instincts and had the system removed at that time.
Even better would have been to not give in to their fears about "people of another color" invading them to commit crimes. The wife's instinct to show compassion on the unemployed strangers outside her gate was the correct instinct. By showing compassion to the needy and trying to reconcile the ethnic and racial divide in their community, they would have shown their son a better way to live and would have ultimately spared his life.