actually, if you want something that will get them comfortable shooting try getting a short stock ar15... they have very little kick, about like a 22lr and are effective out to some serious distances...
That is not a very good weapon to learn with. I high discourage giving them anything that is semi-automatic.
Get them a rifle. Make they put a new shell in after each shot. This way they learn the importance of making your first shot count. With an auto many people rely on the quick reload time and don't put a lot of focus on the first shot.