The frame narrative of "Once Upon a Time," in which someone phones Gordimer and asks her to write a children's story, establishes the subsequent action as having elements of a fairy tale or bedtime story.
The story that Gordimer relates may not be a children's story, but it certainly shares certain features with traditional fairy tales. For one thing, it provides the reader with a moral lesson.
The white couple in the story think that they can protect themselves against burglars by putting barbed wire on the top of the walls surrounding their suburban home. In actual fact, however, the couple's security measures end in tragedy after their son dies in the act of trying to climb over the barbed wire.
What this demonstrates is that simply walling oneself off from the world does not necessarily make oneself safer. Despite all the security measures taken by the couple and their neighbors, the number of burglaries in the neighborhood remains stubbornly high. To a large extent, this is due to the extreme poverty experienced by the majority Black community under apartheid.
That being the case, the suburban couple's illusions of safety and hesitancy toward change—the belief that they deserve and will achieve a happy ending behind their walls—is just the kind of wishful thinking we would expect to see in the children's story the caller in the frame story wants Gordimer to write.