What stops an adult from creating these anonymous tokens and then letting others use them?
But if a kid dies of alcohol poisoning or drunk driving, you can certainly get in serious legal trouble. Those two things (not wanting kids to be harmed by alcohol, and not wanting legal trouble) stop a very large number of adults from giving minors alcohol.
I also would personally prefer we not destroy the internet in pursuit of that goal.
You put severe penalties on the crime, then you catch people doing the crime. Offer a reward fo catching people, and I'm sure a few kids will hand people in for the reward. They'll be able to prove they got a token from someone (as they'll have it), then we investigate.
Require a token to be provided by the requester that is used to sign the response token so its limited to a single use