the bugger as easily as Han Solo fried Greedo. But beyond that, if I am at a safe enough distance that I can avoid getting hit at first, then I would take cover and call the police, while trying to keep the guy pinned down with some warning shots. Once the police arrive, they can take him out from the side.

However, if the guy starts threatening to shoot the child right away if I don't come out and let myself be shot, then all bets are off. I do my best to try and shoot the guy without killing the child, or communicate to the kid somehow to duck on my signal.

Just typing my train of thought here... another option would be to fire an initial non-fatal shot (like through the a leg of both the child and shooter), and then when the guy drop to the ground in agony, move in quickly and finish him off. Odds are he won't be able to keep the child hostage when he is grabbing at his leg and screaming in pain.