More Kids now days lie to their parents or sneak out at night, Going to a party. If the parent let their kid drinking with their parents is more likely the child not going to sneak off or not have a reason at all. They would ask you if they could go to a party and you'll trust them because they're responsible enough.
The reason we have the drinking law at age 21 because when your ageing especially when your a teen you're brain is still developing. As a teen your brain hasn't fully developed the part of the brain that tell you right and wrongs. Around 25 the brain has stop developing, And around 21 is when the right and wrong of your brain is developed. The reason that drinking is bad for ages under 21 is because when you drink it stunt your brains developing process.
There's a reason that drinking laws are put into place. Also, How would that stop kids from sneaking out? Not because I care that much (I kinda care), But because it's illegal. Alcohol is bad enough for adults. It's kinda like going "HERE, MY 14 YEAR OLD SON, SMOKE SOME POT! I'M ALLOWING YOU TO! " Just because it's YOUR KID, Doesn't mean you can do whatever you want with them. Fact of the matter is, The government is basically in complete control over you and the rest of society.