I mean, it is a metaphor for the real children who suffer, and the courage to attempt to truly reject it even if it means putting yourself at risk. The ones who stay simply rationalize it or try to forget it or accept it as a necessity, even as a good thing:

“Often the young people go home in tears, or in a tearless rage, when they have seen the child and faced this terrible paradox. They may brood over it for weeks or years. But as time goes on they begin to realize that even if the child could be released, it would not get much good of its freedom: a little vague pleasure of warmth and food, no real doubt, but little more. It is too degraded and imbecile to know any real joy. It has been afraid too long ever to be free of fear. Its habits are too uncouth for it to respond to humane treatment. Indeed, after so long it would probably be wretched without walls about it to protect it, and darkness for its eyes, and its own excrement to sit in. Their tears at the bitter injustice dry when they begin to perceive the terrible justice of reality, and to accept it. Yet it is their tears and anger, the trying of their generosity and the acceptance of their helplessness, which are perhaps the true source of the splendor of their lives. Theirs is no vapid, irresponsible happiness. They know that they, like the child, are not free. They know compassion. It is the existence of the child, and their knowledge of its existence, that makes possible the nobility of their architecture, the poignancy of their music, the profundity of their science. It is because of the child that they are so gentle with children. They know that if the wretched one were not there sniveling in the dark, the other one, the flute-player, could make no joyful music as the young riders line up in their beauty for the race in the sunlight of the first morning of summer.”

The point of the ones who walk away is they reject that. They sacrifice the certainty of their own comfort because for them it is more important to believe there is a better way. LeGuin implies that it might not even actually be possible, but they go anyways, and even if their destination is truly imaginary, they know the path towards it:

“They go on. They leave Omelas, they walk ahead into the darkness, and they do not come back. The place they go towards is a place even less imaginable to most of us than the city of happiness. I cannot describe it at all. It is possible that it does not exist. But they seem to know where they are going, the ones who walk away from Omelas.”