How clever would a kid have to be to sneak back a key? No one is watching every single one of thirty kids.

Besides, that is not really the issue. No one should have to police thirty kids- and if the adult hadn't provided alcohol, they wouldn't have had to be policed.

The bottom fact is that no adult has the right to break the law - with the risks attendant on this one - with MY kid. If that had been my daughter at that party I would be furious with the host.