Children get upset about all kinds of things, and it's important to help them understand and resolve their emotions, no matter how silly it is.
Eighty cows is a minor inconvenience at worst and like four stacks of steak at best.
So I feel like the confusion here isn't just coming from how to handle the griefer child or how to get the cows out of the house. I think it's more to do with the novelty of the situation.
Why is the child upset by this? Does he not like to kill cows in the game? Is there something preventing him from luring the cows out of the house? Was he just unpleasantly surprised by it and hadn't thought through whether or not it was a big deal? There's a lot of layers to this.
Or maybe this guy just never played Minecraft.