While I understand the Omelas concept on an emotional level, I've never been able to find any justification for walking away. First of all, let us assume that the utopia of the story is not a sickly-sweet harps-and-halos utopia, but actually an ideal society complete with pain, loss, and risk in their proper measures to achieve the optimal growth of human knowledge, achievevment, and spirit. Let us also sssume that the people who leave Omelas will go somewhere where it is likely that they will live a poorer life, excluding any psychological effects that leaving might have on them. Thus, staying is an unquestionable good, though it may be balanced or overruled by other factors, such as the suffering child.

Now -- if you leave Omelas, what does that achieve? Does it reduce the child's suffering? Does it make any difference at all? If not, Then I submit that leaving Omelas is not only futile, it's an act of selfishness towards everyone in the city, including the child. It is a simple averting of the eyes, petulantly pretending that what is unseen is not.
In our world, people often boycott companies that do objectionable things (such as keeping children in sweatshops where just such conditions prevail). They will go so far as to avoid activities that don't even give the company any money, such as refusing to watch television programs they have sponsored. But all of this is directed towards showing disapproval, isloating the wrongdoer in hopes of provoking change. In Omelas, there is no one to shame and no hope of changing, ever. We're talking about magic, not economics.

If a walker really cared about the child, they would not hide from it -- they would do one of two things: 1) strive, against all odds, to learn how to ease its suffering, or 2) make sure that its suffering, the suffering that created Omelas, was not in vain. In the first case, where better to carry out observation and research than Omelas, where one need never worry about petty bureaucrats cutting funding or random sociopaths cutting one's throat? In the second, which may seem selfish, simply reverse the picture and ask which is worse: A child who suffers so millions might live great lives, or a child who suffers for no reason at all?

The ones who walk away from Omelas have morals, but morals that are hypertrophied and inappropriate, rigid rules of conduct that do not work towards any human good.