There are many, many ways to effectively teach your girls to play competitive tennis, effective doubles play, and quite frankly, teach them to become champions every year.
In addition to the good posts that have been offered, I recommend a few additional coaching tips: (For a more comprehensive coaching manual, check out my book Coaching Mastery which I outline the ways my father, National Coach of the Year, Bruce Smith, and I developed the winningest high school team in the country for 22 years! The book is available here at Tenniswarehouse.)
1. Toss and Block drills: develop the volley by spending 5 or 10 minutes a day first doing these drills: pair up players one with a racquet, one without; one tosses to the other player's backhand volley, (or forehand, or half volley or even groundstrokes with a short stroke), the player volleys the ball back to the tosser focusing on the form and the hold. (Obviously, teaching them the grip, stroke pattern, etc, is important before executing the drill.)
This drill alone allows players to work on technique as opposed to hitting the ball into the court. (When players hit balls over the net to targets, they are more focused on the outcome instead of the execution.) The tossing player can hold up their hand after tossing as a target for blocking volleys, or other shots, to aim at as they develop more feel and control. This drill is just as good for the tosser as they are trying to catch the blocked volley, developing their own hand-eye coordination of tracking and waiting to catch the ball...just as we want to do with the volley.
You can do this drill with over 50 players on one court. Add some music, have them switch after 15 or 20 hits see who can hit and catch the most in a row by having a contest...developing greater focus and pressure. (Have those who drop a ball sit until you have one pair left standing.)
The players are only about 10 to 15 feet apart, but you can separate them more to develop firmer volleys as well as working on controlled groundstrokes with this simple drill. (Developing a short backswing, and holding the follow through on groundies is a great practice!) You can teach them to break their swing down into 5 segments, 0 being no backswing, a 1 being a very short backswing, etc., all the way to a full backswing as a 5. Follow through is also designed the same way. You can tell them to do a "1-3" very short backswing with a med followthrough. Or a 3-1 or 1-4, etc. Getting them to feel and know and control the length of their stroke.
There are hundreds of drills the can be used to develop strokes, footwork, swing patterns, strategy, etc., (I've included my fav. drills in my book along with the great Ken DeHart who contributed 50 of his fav. drills in the book.)
Good luck! Having coached 28 seasons of girls and boys tennis teams, I can tell you that it can be a lot of fun and the rewards are great for both you and your students!