I don't think the Demon is trying to destroy Sam and the other children -- he's careful not to hurt Sam in the confrontation in the S1 finale, and it would be easier to kill the babies than their mothers, if that's what it wanted. But they have power (of some kind, from wherever) and it wants to use that.

I think the power can be used against the Demon partly because that's how the story always goes, and partly because in Salvation the fire started in the baby's crib, rather than from the ceiling. So the mother's deaths are used to somehow mark the babies (?), which makes them more valuable or less dangerous (or both) to the Demon.