Personally I think you can read all the books and get all the information you want on how to fit a gun (which is not a bad thing)
However perhaps the best way is to take the kid or any new shooter to a large well stocked gun shop and stay there trying till they find something that fits them and they like.
I did this with my wife, and the gun shop was only more than happy to help, they seem to do that with new shooters.
After about an hour the deal was done, the only mistake I made was being a woman the wife would also gravitate to the most expensive shotgun that fit her.
I kept trying to get her to get the Mossberg 500 youth, but she insisted the Browning BPS Micro fit here better...she won..