I would suggest that the entire thought experiment of Omelas is a metaphor for real-world issues and realities. To give an example, consider the abuses and inequalities of global capitalism and the ways in which corporations (and consumers) have profited off of the exploitation of workers, or the degree to which the entire developed world has profited off of the exploitation of developing nations.
These are only examples, but the fact remains that the entire course of civilization itself has featured a long history of exploitation, inequality, and abuse. Ursula K. Le Guin's thought experiment takes that long history and, in a sense, reduces it to a single innocent victim, on whose suffering the happiness of all society rests.
With that in mind, even when it's safe to say that most people would like to think they would not be complicit in the suffering of the child (even if their own happiness would continue to rest on that suffering), an argument remains that our own history as a civilization and society paints a much more cynical picture regarding our collective nature.