I'm sick of journalists writing about a law without either citing the law, linking to it, or directly quoting it.
Had to find this myself:
The practice of a parent or guardian who chooses for his child treatment by prayer or spiritual means alone shall not for that reason alone be construed to have violated the duty of care to such child.
https://legislature.idaho.gov/statutesrules/idstat/title18/t18ch15/sect18-1501/
Edit: thinking about this a bit, the text says "for that reason alone". I would argue that these parents do not meet that bar because the children actually are harmed. In other words, arrest them and medicate the children not because they are attempting faith healing, but because they are attempting faith healing plus it isnt working. If the faith healing works, then the duty of care hasn't been violated.