First of all I do not see how learning how to throw a curveball teaches one how to pitch
Another aspect which never seems to pop up is the different mound to plate distances the kids pitch at--every time you move the rubber back the kid has to klearn how to throw the curve all over again. Why not wait til you get to the 60 ft 6 inch distance ?
Thirdly as Callaway noted his son never learned how to pitch--having three pitches all at the same approximate speed does not a pitcher make
At 9 years of age the boy should be learning proper mechanics; location and throwing his fastball with various grips to give the ball different movement.