But that's not what justice is about. It's not about making people uninvolved in a situation *feel* better.
Then neither should you be concerned with the parents feelings or whether they've suffered enough already. Justice isn't about feelings, as you say.
Prosecting these ignorant parents will/should deter other parents from allowing their children to go to all night drug parties.
This assuming of course they actually broke a Washington state law. As I suspect they have, as it is a liberal nanny state.