In "Once Upon a Time" by Nadine Gordimer, what is signified by the wall, the boy, the maid, the wife and the mother-in-law?

Given the way in which the story is framed by the author, and given the very short length of this story, we can safely infer that many of the objects and perhaps even the people within the fairy tale can be meaningfully interpreted as symbols. Let's look at each:

The wall: Probably the most important symbol in the story, the wall represents separation and intolerance, the "closing off" of one's mind. When you place this story in its historical context, the symbol becomes even more important: the author seems to be hinting that the social separation of apartheid cannot be tolerated, that it only leads society to destruction.

The boy: When you consider how he gets ideas from his reading, tries them out, tries to get out and over the wall, and ultimately is destroyed, you might argue that the boy represents some or all of these ideas: youth, open-mindedness, experimentation, exploration, reason and rational thinking, and the striving for openness and equality.

The maid: Take a look at every time the housemaid is mentioned, and notice how she's never just "the housemaid." She's always the "trusted housemaid" or "the reliable housemaid." The implication is that the family believes that most people the same color as the maid are not trustworthy. Combined with the fact that the family's "YOU HAVE BEEN WARNED" sign is carefully constructed so as to not reveal the race of the pictured intruder, we know that they're eager to distance themselves from the notion of racism. The maid, then, possibly represents superficial, ineffective, or insincere efforts to eradicate racism.

The wife: She's the embodiment of both positive ideas (compassion, love, and the desire to protect one's family) and negative ones (excess, complacency, ignorance, obliviousness).

The mother-in-law: As the closest thing that this fairy tale has to a villain in human form, and the oldest character, this "witch" might represent tradition, or the blind adherence to tradition. (The family simply does what the mother-in-law says without question and accepts the gifts she gives, again without thought to their possible consequences.)

Notice how I kept saying that something might represent something else, maybe, possibly, could, and so on. Unless you can find an explanation straight from the author about what's supposed to represent what, you really can't say for certain what one thing symbolizes; you can only make a good argument based on what you see in the text and what you know about the larger context surrounding the story (like the author's biography, where she lives and what was going on socially and politically at the time, and so on). 

Complicating matters further is that, when we’re talking about literary symbolism, any particular thing usually represents more than one other thing. (A story in which every object only represents one definite thing is an allegory.) Other readers may be able to think of many other possible answers to add to this list!