The answer depends on your personal point of view and the kind of person you are. Are you the type of person who could be happy living in the "Utopian" town of Omelas knowing that a child must suffer in order for you to be happy? Or, would it be impossible for you to be happy under these circumstances? This is the big question! What price are you willing to pay to have the ideal life? Is the happiness of thousands worth more than the extreme misery of one?
Now, remember that you cannot change the life of this child no matter what you do. If you leave, the child will remain where he/she is and will be treated the same. The narrator tells us that it is already too late to help the child, even if you could, because he/she has been abused for too long and is now "feeble-minded".
Those who do leave don't 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." If you do leave, you won't go to a place as great as Omelas, according to the narrator.
You now have all the facts on which to base your decision. Answer the questions in the first paragraph, and you will have your answer.