We do that each week during winter but the pitchers run backwards from catcher to circle at top speed. Most girls can't do 10 when we start in late fall. But spring they can do 3 sets of 12 with little effort.
I was told to hold off on weights until high school. Just try to get her to strong fast and coordinated. I wouldn’t over pitch her at 9 you want to build her endurance. However think about how many rotations are going to to be made On her little shoulders over the course of her career.
The girls are sacrificing front side resistance to run home and get the ball...unless they have poor posture to begin with?? Nothing wrong with the drill if you're still using proper mechanics I suppose.