How does the use of "Happily ever after" in Nadine Gordimer's "Once upon a time" differ from its use in traditional fairy tales?

Nadine Gordimer's short story differs from the usual fairy tale in that her story commences with the phrase, 'happily ever after', whereas in traditional fairy tales this is normally only used at the end of the story. The following quote from the text illustrates this incongruity:

In a house, in a suburb, in a city, there were a man and his wife who loved each other very much and were living happily ever after.

In a normal fairy tale, the story ordinarily commences with, 'Once upon a time,' which, to add further irony, is the title of Gordimer's story.

Gordimer deliberately employed this inversion, not only to pique the reader's interest or to engineer a dramatic introduction, but also to make readers aware of the fallacy of such a belief, just as the characters in our story do. The story warns of the the belief in an idealised existence and informs of the pragmatic realities of life. Nothing is certain or guaranteed. There is, in real terms, no 'happily ever after.'

This incongruity also illustrates that the lead characters, the man and his wife (and their family), lived a life of privilege guaranteed by the apartheid system in which they lived at the time. They were members of the white race who lived lives of comfort and security - privileges granted them by law. It was this protection that made them believe that their futures were guaranteed, in essence, 'happily ever after.'

Because of this, the privileged class felt entitled to enjoy just the best South Africa had to offer at the time. They developed not only a supercilious air, but were also condescending and patronising towards those who were less privileged, as are our characters in the story. When they felt that their security and safety was threatened, the family took to desperate measures, to ensure their further happiness. Unfortunately and ironically, they disregarded life's little iniquities and their hope turned to sorrow - their 'happily ever after' was ultimately destroyed.

In "Once Upon a Time," why does Nadine Gordimer use the phrases "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.