Seems to me all they were doing at the point you hit them was touching the fence. You attempted to cripple them over touching your fence?
Gimme a break. Don't trivialize actions in such a manner that we can show any activity is virtually harmless. This reminds me of a lady I saw on TV who was crying (literally) that her baby was killed for trespassing and the punishment for trespassing was not death. In reality, her boy was killed during a B&E with several other boys and was in the home of a man who defended his family. The boy was unarmed and was trespassing, no doubt. The mother made it sound like the boy was doing nothing harmful what-so-ever, and maybe he wasn't (yeah, right), but from the perspective of the man, the boy and his fellow gang initiates were advancing down the hall toward himself and his family and would not stop until he shot the lead boy. His actions were completely justified and no charges were filed against him.
He did not hit the trespasser for "touching the fence." Did you not read? He hit the trespasser for attempting to trespass again.
That is when I hear the idiots jumping back on the fence. I turn around and get to the fence as two of them are trying to get over the fence. I see two sets of hands and use the only thing I have at the time, the mallet.
Now while I don't believe hitting a guy's hands with a mallet is an appropriate line of action here, the point is that it was done to stop a perceived crime against the hitter. He protected himself and his property in the process against what he perceived as a threat.
Along similar lines, you can't use deadly force against another who is holding a knife. Holding a knife isn't a crime, even. However, if that person holding the knife is threatening your life, you can use deadly force. We can trivialize the situation and claim the aggressor was "just" holding a knife, but that doesn't convey anything about the intent. Sure, the tresspassers were "touching the fence" but it wasn't for the sake of feeling the wood grain, as perceived, but to commit trespass anew.
If you honestly felt your life was in danger, from 4 kids, then that's a different story. It just doesn't seem like that was the case though.
Right, nothing in the OP bespeaks of fear for one's life. In fact, it all sounds rather hokey. Why the cop would allegedly state that trespass was a Class A felony is just off the scale in wrongness. The only way it would have been more wrong is if the cop allegedly said it was a capital crime.
The question is one of whether the actions taken, hitting the hands with a mallet, were actually justified.