"Que viene el Coco" is a verse in a Spanish traditional children song that warns the misbehaving that if they are not going to sleep, the "Coco" (kind of Boogeyman) will come and eat them. This resonates in the series topic of the gruesome murders where children are killed and bitten.
The line, "Que Viene el Coco" is "coming of the coconut" literally, but is synonymously rendered as "The boogeyman is coming".