they dont. either they ban it altogether or they pretend they dont know what's going on.....e.g they go stay at a hotel for a night.
knowingly allowing underage drinking to occur in your home as an adult is a felony in most states. you can go to prison and kids taken into foster care. in some places parents are allowed to drink only with their own children, and even that practice is becoming illegal in more places.
in the U.S, teenage parties get very loud. neighbors call the police almost always. When the police arrive, if the parent to whom the house belongs to is present and underage kids are drinking, the parent goes to jail. so does anyone over 21 who can be proven to have provided alcohol for the kids; while the drunk kids get off with a warning.
pretty idiotic system if you ask me. you danes know how to run things much better.