By calling the story "Once Upon A Time," Gordimer sets certain expectations for the reader. Specifically, the reader expects to be presented with a classic fairy tale story in which good conquers evil. What the reader finds, however, is that although this story has some characteristics of a fairy tale, Gordimer has no intention of letting good conquer evil, as we see through the story's tragic ending. Therefore, she turns the notion of a fairy tale on its head.
The title also has significance in terms of Gordimer's refusal to write a children's story. Remember that in the first paragraph she says that she was asked to contribute to an anthology of children's stories. In addition, one author said that she ought to write at least one story for children. However, Gordimer has no intention of fulfilling this request. By titling her story in this way, in which she sets an expectation for a children's fairy tale story, she makes a protest against this expectation. Gordimer is, therefore, exercising her artistic freedom.
As such, the title acts as a protest. It sends a clear message that she will write what she wants to write, not what society tells her to write.