Having turned 18 close to 10months ago i know what its like to be underage drinking. I found that at the age of 15/16 was the worst as it was almost completely restricted, so being the rebels that every teenager is, went and drank with no idea of what we were doing. Drinking to get drunk purely on principle. I found that the complete taboo placed on drinking by parents makes it all the more attractive as you're not allowed to do it. If you've got kids, don't simply tell them "NO!" as i can promise you, without doubt that you are only fuelling the fire and giving them cravings.
Its like you hearing about an awesome new attraction, then being told you're not allowed to do it, no explanation, simply "i don't want you doing it, its not right". Now what would you do? Sit at home and be safe or risk it and check out what it is for yourself.
Give your kids a bit of information "not that i think you're drinking but...if situation x ever happens i want you to call bla bla bla, ambulances WON'T dob you in, police WILL look after you, we WONT be angry at you"
Its just a small insurance that if your kids do get in trouble that they won't kill themselves, or a friend in the process.
I had fairly lenient parents, not remotely keen on me drinking, but mum being a school teacher knew that i was going to be drinking at some point or another. she made me have the local ambulance number as well as the local cop shops number in my phone as 9 times out of 10 they werent interested in filing paper work and if need be would simply drop kids home, no questions asked.
Tell them not to expecting full well they will!