With the 28, there won't be a change in felt recoil and you can experiment with stock length without committing permanently. For your right handed daughter, the Beretta 391 youth model 20 is a marvelous gun. If you feel that she is mature enough at eight, that's up to you. I'd completely plug the magazine and make the gun a single shot for a while. I know that you will work with her on that.
Kids are more flexible.