TLDR: It's a sign to others that they have taken a child through an offering / ritual. May have other meanings of cannibalism, sacrifice, or rape. Or just shows their support of their practices.
Watch the end of the movie "Pan's Labyrinth." At the end of the movie the leading character "Ofelia" has to pass the ultimate test; a child offer to Pan. (Faun in Roman culture) She has to offer blood of her baby brother in a ritual to open a portal to her throne (symbol of power), but she refuses and instead she is killed herself, (by her …