The falling action of a story occurs after the climax and ends in the conclusion.
The story "Once Upon a Time" actually contains two stories. First, there is the frame story. This is the beginning where Gordimer explains how she's been asked to write a children's story. She is reluctant to do so. Then, she awakens one night to some unexplained sounds. Unable to sleep, she tells herself a bedtime story. The falling action of the frame story occurs when Gordimer realizes that her house is making the creaking noises. It had been built on top of a mine. This falling action resolves to a conclusion when she determines to tell herself a bedtime story in order to fall asleep.
This bedtime story is the "story within the story" or the "story within the frame story."
The falling action of this story within the frame story (the "children's story" about the family) occurs at the very end. The rising action of this story consists of the family's growing distrust of outsiders and their need to reinforce their prison-like home. This story reaches its climax with the little boy getting caught in the security coil. The falling action consists of the family carrying the little boy's lifeless body into the house.