I think that Le Guin is asking the reader to make critical judgements and about who we are and what we should do.
Le Guin presents a complex and nuanced view of happiness. She presents Omelas as a world of happy, civilized people and "not simple folk, not dulcet shepherds, noble savages, bland utopians." However, their social framework demands evaluation. Le Guin wants readers to consider how the happiness of the townspeople comes at the cost of the suffering child.
Le Guin wants us to ask critical questions about this condition. In this way, the reader bears similarity to the people of Omelas. Would we accept that our happiness must come at the cost of another? Should we ask questions about it, and even take action, such as the people who walk away from Omelas? Should we accept with a heavy heart the institutional framework that maximizes happiness for some and unhappiness for others? Le Guin does not give an easy or direct answer as to what we should feel or do. The one thing that is clear is that we must ask questions. We must probe into the reality that defines our existence and be prepared for what we find.
Le Guin insists that knowledge might lead to some level of discomfort, as evidenced by the ones who walk away from Omelas. There can be no retreat into blissful ignorance. Through the philosophical tone of the story, it seems that if we ask probing questions that allow us to absorb and reflect on the answers, Le Guin's end goal would have been met. She wants us to ask the critical questions about what underscores our happiness and how this impacts others. In raising these questions and showing the courage to recognize the validity of the answers we get, it seems to me that a part of Le Guin's motivation is met in terms of addressing how we should feel about our comforts and what we should do.