This becomes the central issue of the story. I think that LeGuin deliberately configures it so that the reader has to make this assessment for themselves. On one hand, the citizens of Omelas' success and happiness is something that benefits so many. The entire setting is able to partake fully in this element. There is little in way of stratification and division in Omelas, as it is presented as fully open to everyone and anyone in terms of participation. If one pursued the interests of democratic rule or utilitarianism ("Greatest good for the greatest number") in the strictest of senses, then the suffering of one child is the necessary sacrifice and cost of this happiness where so many benefit.
At the same time, LeGuin is not afraid to expose to the reader the costs of this calculus. Readers have to assess what it means to be the result of this decision. In what Mill would call, "the tyranny of the majority," examining the life of the child is something that compels the reader to think and forces a strong commitment to their idea that the child's suffering is secondary to the happiness of many. LeGuin does not force the issue to the reader. Rather, she allows the reader to reflect and choose what decision is to be made.
It is this basic idea that forces the issue in the reader as to which option is the "correct" one. In the end, there are no answers offered because there is only challenge faced. The fundamental theme is how one goes about determining the needs of the many as opposed to the needs of the few. LeGuin does a great job of posing this philosophical and political question in a literary form.