Answer:
I believe that the correct procedure in a case like this, and of course, depending on the whole age-restriction, and since it is my house, and not public property, would be to request that they put the bottle away, and not drink it in my house. I would also try to talk the girl into not drinking the bottle, neither in my house, or anywhere else. I would try to make them understand the danger of what they are about to do.
However, if they are all underage, I would also threaten that if they do not put the bottle back where it came from, or gave it up, I would seek the proper people to let them know of the situation. I would make sure that they understood my position on drinking, and especially when the age restriction is being blatantly violated. And I would let them know that my home is not the place to choose to break the law either.