I think the key thing that most people are missing here is that the parents were letting their kids have a party with alcohol. I doubt they would have gotten in such trouble if they were merely serving their own children alcohol in their home. However, when you open something up to other peoples' children, it's not a shocker to see that someone gave an anonymous tip. It was probably from a parent who found out what was on the agenda, and didn't allow their child to go/the child wasn't invited. Or, someone who wasn't invited to the party called in the tip.
While the sentence is a bit much, the parents should have known better than to serve alcohol to minors at a party. That's just begging to be arrested.