Consistency

I assign my ethics students both "Famine, Affluence, and Morality" and "The Ones Who Walk Away from Omelas" to read.  It is perhaps predictable that they mostly don't think they should be giving money away, but that they do think people should, and in some cases that they would, walk away from Omelas.  But how can it be right to sacrifice dozens or hundreds of children to painful deaths to keep one person moderately happy, and not right to subject only one child to a miserable fate to keep a whole society ecstatically happy?  I think this combination of views cannot be tenable.