To be a little more clear on my personal view, for what it’s worth:
The likelihood of a prosecution for stam providing alcohol to a minor on purim, while conceivably MIGHT be prosecutable (though as Schwab pointed out, unlikely) is extremely low without any other problems. One exception to this might be large parties held on college campuses by various Jewish groups. I’ve known a few of them to get raided, though as far as I recall the only people who got in trouble were the underage drinkers, and there was some story about the parties having gotten so popular that many non-jews were showing up for the free booze. Be careful about that, though I doubt anyone here is likely to be involved with such an activity.
The likelihood of action is greater where something bad happens as a result of the minor drinking. There are a number of ways through which one could be held liable, even in the absence of social host laws. At the very least, the lesson is to control what goes on in your home, and what might happen after people leave. Be responsible and cautious. Liability is nothing compared to the grief you will feel if the child that leaves your house kills someone, gets killed, or both (c”v).