How did they know the masks were for mocking child sacrifices?
It isn't a sacrifice if you don't care about or if you despise the victim.
For example, when I bled my father on the horrid altar of the polytropic, solar God - that God of infinite subtlety, He of threes whose reach no man or beast or thing exceeds - I wept buttery tears that fell fat and with the crimson intermixed. This too was a sacrifice of grace and native wonder. Only those things most precious to us are fitting to be given up, so it was ever thus.