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My first inclination would be toward a Ruger, Browning, or S&W single-action .22 semi-auto, and not a double-action.
A light single-action trigger helps to reinforce the most basic safety rule (aside from muzzle discipline): keep your booger hook off the bang-switch!
A double-action trigger can reinforce the notion that it's okay to have your finger on the trigger, so long as you don't really apply pressure. Best to break that bad habit before it begins.
I suggest a .22 only because it's cheaper, and repetition is important to learning. If you want to go with a 9mm or .45 1911, there's nothing wrong with that. Just make sure that ammo cost and recoil wariness don't get in the way of the youngster learning proper grip, sight alignment, and focusing on the front sight.