I recently read that, besides wanting to sacrifice babies for the adrenochrome, that also, the satanists choose to sacrifice the youngest, most innocent aged children because the children are the purest, thus the most alike to God.

Thus, because the satanists hate God so much, in their evil, twisted minds, that is the most direct way of hurting him and pleasing Baphomet. And of course there's the adrenochrome aspect as well.