I really like a tlike a single shot 22 rifle (like the Savage Cub) or a single action revolver (like a Single Six). The SA pistol is the favorite of my kids (even more than the Mk II). I like these choices not only because of the "safety" but for 2 other reasons:
+ it slows things down in the beginning. You can always switch over to a semi to "have fun", but early on they need to meter shooting with listening. Remember that early on, there is a lot of info coming at them.
+ it teaches them how the gun functions. When you have to load, manually cock and unload each shell, there is learning.
You can intersperse the semi to keep things interesting.