The children in the train car are the most straightforward symbols in the story. They represent society as a whole, and their views are intended to be synonymous with the moral ideals of the larger real-world public. In other words, just as the children expect exceptional virtue to be rewarded exceptionally, society expects exceptional virtue to be rewarded exceptionally. The whole purpose of the story within the story, the one the bachelor tells to the children, is that exceptional virtue can be a danger. The children's interest and surprise in this revelation serves as a symbolic prediction of how an audience will respond to the short story.
The Good Girl
The good girl in the story within the story, who is referred to as "horribly good" on a couple of occasions, represents less a specific person or demographic than an ideal. Her sole purpose in the story is to function as the symbolic manifestation of that ideal -- in this case exceptional goodness. Her actions and thoughts are very generic for the simple reason that ideals do not have personalities. Instead, the girl serves to demonstrate the benefits of virtue and how those benefits can lead to serious consequences that would not otherwise befall a less-good person.
The garden is an unusual element of the story. It is meant to represent the rewards of virtue, but it does not embody the traditional image of an Eden. That is to say, it is not a standard-variety paradise. The pigs have eaten all the flowers, so the traditional symbol of beauty is gone and in its place are pigs, suggesting the garden is more of a place of interest rather than of pleasure. Regardless, the garden symbolizes the gifts bestowed to those who are virtuous and forbidden to those who are not. This idea of an exclusive reward for goodness is an idea possessed by the children on the train and, therefore, by society as a whole.
The pigs tie closely into the aforementioned symbols through their representation of moderation. When the good girl attracts the attention of the wolf, she is attracting it away from the pigs. This symbolizes the primary theme of the story -- that being exceptional can attract both positive and negative fortune. The pigs represent the ideal of moderation, which is why they are able to escape the danger of the wolf. It is an albeit unorthodox symbol for mediocrity, but its purpose is well-served.
The wolf simply represents misfortune. Its role in the story is to upset the fundamental understandings of moral justice, and it does this through the devouring of the good girl. Had the girl not been exceptionally good, the wolf would have eaten one of the pigs. But because something exceptional caught its attention, those that were moderate were spared. The wolf, just like the girl and the pigs, is an embodiment of an ideal. Saki presents a view of the world -- that exceptional virtue is not always rewarded exceptionally -- and then all characters, including the wolf, act in accordance with this newly established law of nature. They are all merely symbols for the ideals needed to perform this rejection of a common public perception.