What purpose does the opening of the story serve?

The opening of "Once Upon A Time" is unusual because it begins with the author's refusal to write a children's story. She says that she wants to preserve the freedom to write whatever stories she chooses, not the stories that other people expect. In addition, the beginning of the story is also unusual because it comes into her mind when she hears some strange noises in the night and cannot get back to sleep.
The purpose of these two opening points is to inform the reader that this story is not going to be a traditional fairy tale, as we might expect from the story's title. In fact, the narrator is telling the reader that she intends to create the very opposite of what is expected of her. She is, therefore, setting out her intention to create a different kind of fairy tale, one which does not follow the traditional conventions. As we see from the story's character and events, this is exactly what happens. Gordimer writes a story that is sinister and unnerving, the very opposite of a Disney-style fairy tale.
The opening of the story, therefore, foreshadows her unconventional and resistant approach to telling a children's story.