What ironies do the characters display in actions in "Once Upon a Time"?
The massive central irony in this tale lies in the way in which the father and the mother think they are taking various measures to protect themselves and their son from harm, whereas what they are actually are doing is nothing more than threatening their own lives and the life of their son and perpetuating the fear and unrest that exists in their homeland. Note the following quote that explains how this process operates:
But every week there were more reports of intrusion: in broad daylight and the dead of night, in the early hours of the morning, and even in the lovely summer twilight—a certain family was at dinner while the bedrooms were being ransacked upstairs.
The fear of attacks, robbery and murder therefore prompts white settlers to invest in ever more technical and expensive safety equipment that fundamentally changes their lives as open houses are turned into gated communities that are locked away from everybody else. The way in which the ultimate security answer--the dragons teeth--promises total protection whilst resulting in the probable death of their son makes the irony that Gordimer is pointing out clear.