In the story "Once Upon a Time," why does Nadine Gordimer choose the words "happily ever after" and "once upon a time?"

At the beginning of the story, Nadine Gordimer gives the frame story. She tells the reader that she has been asked to write a children's story for an anthology. Initially, she does not accept the offer, replying that she does not write children's stories. She doesn't like the idea that she "ought" to write something. However, her creaking house and worries about crime in the area prevent her from going back to sleep. So, she begins to tell herself a bedtime story. This is to be the so-called children's story that she mentioned earlier. This is her children's story within the frame story. 

Sticking with the traditional fairy tale form, she uses phrases like "once upon a time" and "happily ever after." She describes a family in a crime-ridden town, similar to her own. In order for them to "live happily ever after," they continue to improve the security of their home. Eventually, a once simplistic home becomes more like a prison designed to keep criminals out. In the end, the little boy, inspired by a fairy tale from a book his grandmother (referred to as a "wise old witch") had given him, tries to get through the security coil. He is killed and the story ends. Gordimer uses those phrases ("happily ever after" and "once upon a time") because she'd been challenged to write a children's story. The phrase "happily ever after" turns out to be quite ironic (situational irony) because the story ends in tragedy. 

Gordimer's story alludes to the transition in South Africa from the apartheid to an unknown future. The family in the story claims not to be racist but they do worry about people "of another color." Gordimer uses the frame story and the story within the story to allude to South African culture (1989 at the time it was published) and how it is not a fairy tale situation. Therefore, the "happily ever after" phrases are meant to be ironic.