It not only must "fit" them, they need to be able to "manipulate" it. Load and unload, and cock it if necessary. Children have trouble racking the slide on most semi-autos. It can become frustrating.
Part of me say, get the most accurate gun that fits the other criteria. They'll enjoy shooting a lot more if the gun shoots well.
The Buckmark is a good choice as is a Ruger MKII/III. I'm not convinced that single action is so much safer for them to operate. I've observed people with small hands waving single shots around in trying to cock the hammer. That said, you may also look at some quality da/sa revolvers. They probably can't shoot DA but the lock time on them is usually shorter than many single actions, which can improve accuracy.