Kick or force the dog clear of the child and shoot it with your handgun, where ever the incident might be taking place. Option 2 is your knife, only if the dog won't let go of the kid for you to get a clear shot. Option 3 is whatever blunt instrument might also be handy. I got out of the vehicle with a handgun on a busy city street to shoot a run-over dog that was in agonizing pain. I'd do it again tomorrow, and the police could write me a ticket if they had to.
