Hi Renee. This happens to be my client demographic! 🙂 I train girl athletes of all ages. I think that 14 year old girls can definitely benefit from strength and conditioning for their sport. For these girls, keeping their upper bodies (particularly shoulders) nice and strong is a must (of course along with their core and legs – hmm, did I leave anything out 🙂 I wrote an article for PTontheNet on Shoulder Exercises for Female Throwing and Overhead Athletes, you can access it on their site if you are a member.
For your girls, I’d use some ladder drills (always fun!) to help with their agility; relay races for their speed; ankle cuff side shuffles and Monster Walks (for those hamstrings and glutes), and tube resisted double external rotations, front and lateral shoulder raises (for those shoulders). There are many, many more (for example, I actually use a water gun “fight” to help some of the young girls I train with their agility (I can better explain how if you want to send me an email – see below).
I hope that this gives you some ideas! Most of all, have FUN (I know you, and they will!) – very important for this demographic.