What stylistic devices create the atmosphere of children's stories in "Once Upon a Time"?

Gordimer doesn't really get to these devices or her "children's story" until she begins to relate the bedtime story. She begins this story in a very general way, and this serves to suggest that this story is about "any" family in any place or time. It is therefore very accessible to many families:

In a house, in a suburb, in a city, there was a man and his wife who loved each other very much and were living happily ever after. They had a little boy and they loved him very much. They had a cat and a dog that the little boy loved very much. 

Note the simplicity. This is a simple description of a loving family living "happily ever after." This phrase clearly echoes the most famous final line of many fairy tales. The most famous opening line, "once upon a time," only occurs as the story's title. Still, this opening paragraph about the family almost conjures that phrase in the reader's mind. There are typical fairy tale phrases and a simplistic style, both of which are designed for younger reading audiences.
The phrase "happily ever after" is repeated in this story. The family continually tries to live happily ever after. As they increasingly try to transform their house into a fortress, they believe the security the fortress provides will enable them to live happily ever after. The story does end with some gruesome, tragic irony. This seems contrary to what a children's story should be. There are classic fairy tales with gruesome events, even though many of these end with "happily ever after."
Gordimer is doing a lot more than presenting a children's story. She is indirectly addressing social tension in South Africa through this story and the country's uncertain future. She and other South Africans surely wondered about their future as the country struggled to end apartheid. They must have wondered if and when they would or could live happily ever after.