Your going to run into an issue with size. I had to go to an AR15 style weapon for my 8/9 year old. Traditional guns do not fit her including the youth models. She used a 223 last year and this year I built her a 6.8 to hunt with.

You can probably start them on a regular gun and make it work. But most likely the gun will not fit them and they will pick up bad form. An AR lets you set it up to fit the child up a little better which will help prevent bad habits. I work with a guy that has done just that. His daughter can shoot but its mediocre. The last trip to the range I let her shoot my daughters rig and she was able to do much better as she made the gun fit her instead of trying to make her fit the gun.

Just think about it. If your not comfortable with a gun and how it fits you most likely you don't shoot to the best of your abilities.