I'm going to assume you know proper throwing mechanics (if not,
http://www.sportskool.com/sports/softball will help them and you get started).
It's just making them stand in 2 lines, throwing to each other and not letting them side arm. For a righty: As soon as one side catches the ball, they correctly transfer the ball to their throwing hand, point their elbow/glove and their left hip to their target, right arm bent at 90 degrees, palm/ball facing behind them and freeze. Over the top throw, freeze at follow through. If you break it down and make them freeze, they don't have the pressure of trying to throw hard and can concentrate on good mechanics.
If they do their drills religiously, it should take @ 6-8 weeks to see great improvement. In my experience, 6-8 months before they have control, are throwing properly, and back to throwing hard.
Your task is hard since you're coaching rec ball. You don't have the time with the girls that TB coaches do, but I applaud you for trying to fix the problems. You may also want to enlist the help of some older TB girls that you know can throw. The girls seem to like it.