Parents should have friendly relation with their children. Don't expect children to be open/honest with parents. Parents should from time to time ask their children whether they are in relationship or want to enter a relationship. Then if yes discuss about the relationship but not interrogate. Don't act like a detective or interfere. Quite natural to have a crush/infatuation in teenage which can lead to a relationship. Parents should accept that it may be one of the need of children.
After all parents are mature/responsible and children are not. Parents should be proactive to avoid problem at a later date.