It's a very sad case of parents who love their religion more than they loved their child. It's why the ancient Phoenicians (at least the nobility) sacrificed their first- born children to Moloch, and why countless thousands of parents of my great-grandparent's generation kept a stiff upper lip when the telegram arrived informing them of the death of a loved son in the trenches- belief in the Greater Good. And of course there's no denying that sometimes there can be greater goods.
Our own NHS sometimes denies treatments which could prolong a sufferer's life from a belief that the cost outweighs the patient's right to life. That's a different sort of Greater Good, and I'm not sure why economic belief should be put on a different plane from religious belief.
The child is dead; the parents have been rewarded for their belief. Perhaps they will question their God, but probably not.
The case is quite different from that of vaccination, as no other child is at risk because of their actions.