I see the value in "zip balls" for teaching pitches. It would be a great tool for teaching how to "spin" the ball.
For teaching batting, I don't think the zip ball makes much sense. You can do the same thing a bunch of whiffle balls, ranging in size from golf ball size to regulation softball size. A "bunch" means something around 100 whiffle balls. Kneel down about 20 feet in front of the girls and toss them quickly a bunch of pitches. Have the rest of the team pick up the balls and return them to you. You can go through a lot of kids very fast and you don't have to worry about getting hurt by a batted ball.
IMO they are perfect for the yougest players because they have the weight of a real ball, but are of a size that even a kid with small hands can grip properly. It's hard to teach a kid proper throwing mechanics when they have to cup the ball to keep it from falling out of their hand. I used to use baseballs for younger kids, these are much better.
Sluggers - I do use the whiffle balls, I don't have 100 of each yet, but I am getting there. I have told the girls the will be hitting M and M's by the time we move outside. It is great for hand eye coordination.
Bass - yes the would hurt, that is why they have the "dead cow" on the end of their arm! We only use RIF #1 balls for pop up training, I see no benefit for a smaller ball when trying to catch and control a pop up.
SnocatzDad - That was my thought, as long as they get the proper mechanics down they will be OK. The will need to transition to the 11" we use, which all the girls have played with before.
Thanks again, this place will be a great help this season.