This, of course, is very hypothetical. A sane man anywhere in today's world would have to have a sense of wrongness about child sacrifices.
At this point in the hypothetical, I fear we would be forced to make too great an assumption. You either assume that the person has *no* moral qualms within his consciousness whatsoever about sacrificing children, or assume that he *does* have some sort or presence of internal convulsion, either great or small, that suggests the act is wrong. I think that someone of the Judeo-Christian fold would tend toward assuming that in all humanity there is the law of God imprinted upon the soul that, at the least, would send off at least some sort of internal moral incongruity in sacrificing a child, at some level in the individual.
However, that said, I would just back up to the general statement that the Catholic position regards a person's willingness to not only follow their conscience, but also suitably inform that conscience to the best of their abilities. This becomes a subjective matter that is left up to God to determine (that is, how culpable a person is with regard to their own participation in the formation of conscience). Discussing this relative to a child-sacrificing pagan seems to be a moot point since we'd be trying to do it in a vacuum. All a Catholic could say with regard to that individual's salvation would be that it would be possible for God to save the person.
Salvation is a matter of how we respond to the truth we are given in life. Only for those who have encountered the fullness of truth in Christ is a formal proclamation of belief in Christ necessary.
The web site is located at http://www.cc.columbia.edu/cu/augustine/a/salvation.html