You know, everyone is different, and I know not everyone feels comfortable handling things like I would. But if the same thing happened to me, I would have beaten the adult rather severely. 
Perhaps I am backward in my thinking, but I find that the very real threat of violence is a very good deterrent. And although you cannot legally use deadly force to protect property, I believe that you can use non lethal force to protect property. Perhaps the prosecutor can correct me on that. 
Nonetheless, several crimes were committed. Trespassing, theft by unlawful taking, animal cruelty, unlawful transaction with a minor(s)... your state may call it something different. Did he have to enter a coop? Your state might even call that burglary. I'd consider a chicken coop a building. What about you Mr. Prosecutor?
Bottom line is don't let that fly. He's liable to think that you're chicken, then you'll be at the bottom of the pecking order.