A parallel could be drawn between the child in Omelas and the existence of extreme poverty in the midst of wealth that is a hallmark of Western societies.
It is often the case that people will try to justify such poverty on the grounds that the poor are largely responsible for their own condition in life—that their poverty is a sign of laziness or moral weakness.
Some try to justify the persistence of inequality as a necessary precondition of prosperity. They argue that it is only because we allow a handful of people to get fantastically rich that the economy as a whole is able to create job opportunities, which will ultimately benefit the poor.
Even if that is the case, there will always be poor people in Western societies, not least because some measure of exploitation is unavoidable under a capitalist system. By the same token, Omelas, if it is is to continue to function as a society, needs to have an abused child to exploit.