IMHO, Dennis has the right idea. Chuck Edwards made a 40 cal rifle around a 38" A wt Rice barrel for his son that handled exceptionally well. This gun had a 13" or so LOP and the correct amount of drop to fit his 12 year old son, at the time, but I could shoot it very easily.
IMHO, such a rifle allows room for growth, but will remain a usable rifle for rest of a youngster's life.
God bless