The only reason I suggested teaching kids how to throw change-ups instead of curves is because it seems to take kids much longer to feel comfortable throwing the change up than a curve.
It is very easy to teach most kids how to throw a breaking ball but many kids really struggle with learning to use the same arm action and arm speed for a change up. I also think it is the most effective offspeed pitch. Even more so than a breaking ball.
Yes, good instruction on throwing mechanics is without a doubt, the single most important factor to helping kids learn to pitch. However, in order to throw a breaking pitch without hurting your arm, you need to have solid mechanics for throwing fastballs. If you mechanics are flawed for throwing fastballs (ie, elbow dip, front side flying open...) then you will undoubtedly hurt your arm throwing breaking balls, if you haven't already hurt it throwing fastballs.
The focus of 13 yr old pitching instruction (first year from the 60'6") should be on fastballs and pitching mechanics. And for the purposes of teaching them how to pitch by using off-speed pitches, have them develop a change up and/or 2nd fastball grip (2 seemer). These all use the same mechanics as a fastball and will continue to encourage and promote good mechanics.
And yes, please keep pitch counts down.