As a former defense attorney (now retired) I wanted to share with you some pertinent facts. The religious exemption being debated here, as long as claimed legitimately, is rather solid and generally no prosecutor will file a case. This is irregardless of any social host laws previously mentioned.
On top of that, the “Free Exercise Clause” (and to an extent the “Establishment Clause”) of the U.S. Constitution protect member of a religion (in this case Jews, including underage), from engaging in alcohol (in this case on Purim) as prescribed by their religious authorities. In the unlikely event an overzealous prosecutor did file a case, it would almost certainly be thrown out of court without trial.