“(The law allows underage people to drink if their parents allow it.)”
I’m not a lawyer so I’m not going to give legal advice, in the state I live in a father (not frum) was arrested for giving his son a Mike’s Hard Lemonade at a baseball game. He didn’t even know it had alcohol in it and he temporarily lost custody of his kids. Granted this was an extremely weird case but you should be careful, not all states have the same laws.