Explain how the author uses tension to build up her surprise ending.
First, Gordimer talks about a request that's been made to her to write a children's story. Then she describes waking up to a strange sound. Fearing it might be an intruder, she can not fall asleep. Even after she's confirmed that the noise was her creaking house, she begins a story to help her sleep. Gordimer establishes some tension in this first "frame" story when she describes her fear of a potential intruder in her home.
Then she goes into the story about the family and their own fears of intruders and crime in neighboring areas. As the story progresses, the family continues to upgrade the security of their home. The family continues to hear about more break-ins and more crime. This stokes their fears and they add to the security of their home:
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 -
They hear about more intrusions and they respond with more security. The reader might suppose that an intrusion is coming, in spite of the family's proactive work to secure their house. The increased reports of intrusion, coupled with increased security serves to raise the tension. The tragically ironic (surprise) ending is that the "security" coil is what poses the most danger to the little boy.