You can talk to them because the parents are "guarantors" of the lease. Absolutely you can talk to them.
In fact, being both a parent and an attorney with children in college and hearing from other parents who have "guaranteed" the rent for their college children:
1) the parents are scared to death that they be charged additional monies
2) that their children are evicted - where can they go
3) that they are charged for damages
4) that their children are doing something illegal and you might call the police
So, absolutely call the parents - if you want to see your tenants jump - call their parents.
You will not have any problems in the future with them most likely.
However, you should, for future reference - have them purchase renter's insurance. You can set them up with your local insurance agent and he can suggest some minimum coverages that you can demand they purchase.