I looked into this a while ago for my daughters team. Her team was a little older. At the 9-12 age, they're still just looking for basic throwing, fielding and hitting type skills. They should also be starting to get into some strength training and mobility warmup drills, mostly to get them prepared for their teens. You can build the habits easier now. The most common serious injury in teen girls softball is ACL tears. Strength and agility training early on can prevent it. In your age group, they don't play as rough so it's not so much injurious, but by 13-14 they should be strengthening their knees habitually. It's not too early to get some of your older girls working on this.
There's a bunch of stuff here: http://www.softballperformance.com/softball-drills/http://www.softball-spot.com/
Here are some strengthening tips for preventing ACL injuries.http://www.sbcoachescollege.com/article ... uction.pdf
Other areas on common injury are shoulders, elbows, wrists and back. All these need attention as well to prevent injuries. Like I said, in U12, injuries are not common, it's at high school and college that the girls get aggressive and building strength building habits during the U12 years will help. As you can imagine, getting a 15 year old girl to pick up a weight is a lot harder than if you start her at 12.