If they have to "obtain" a WI hunting license in order to carry their firearm unsupervised, they wouldn't have to be actively hunting. All they would need is the license itself.

Even then, given that he was surrounded by other people (of which most if not all were over 18) who were doing the same thing he was, one could easily fulfill the requirement of supervision. Nowhere does the law state that a "responsible adult" must be the owner of the gun or be a person that was known by the subject prior to the incident invovling the gun.