Parents need to know that kids under the age of 21 should not be present at a party at which alcohol is being consumed. That's where drunken people congregate, and it would be gigantically unfair if those drunks victimized them. Even if they aren't drinking and they are within arm's reach of alcohol, they are by law in possession of alcohol and can be charged. If a person under the age of 21 does drink, he or she shouldn't try to drive. A minor with a blood alcohol content of 0.02 can be convicted of driving under the influence.
Kids can reach that level with one beer, one shot or one wine cooler.
When kids drink, they lose their inhibitions and they're more likely to take risks sex, crimes, DUI, thefts, vandalism, negligent vehicular homicide. Ultimately, though, the responsibility belongs with the parent. Sometimes it seems like parents are afraid to parent, afraid that big brother is going to come and take their children if they discipline them. That's not true. In some cases, kids have convinced their parents they will turn them in if they get spanked, and the parents have bought into that logic.