A few years ago an article titled "Throw like a girl? With practice, you can do better" was printed in various news sites. It featured the Harvard Softball Coach and her approach to teaching girls how to throw (not like a girl).
Having the girls throw to each other is a good way to start every practice, and proper technique can't be over-practiced. For example, watch the warmups of pro athletes - still working on proper technique and mechanics. It's also why driving ranges are always busy ;)
Start with wrists only:
1) Position the girls in two lines, partnered up, just a couple of yards apart
2) have the girls hold their glove arms so that the forearm and glove is perpendicular to the ground.
3) Rest the throwing arm elbow on the glove.
4) Have then throw the ball with their wrist only
This teaches the girls to get their wrist involved in the throw, and strengthens the motion.
Throwing from a kneeling position:
1) Move the two lines further apart, maybe 20 feet or so
2) Have the girls kneel down on their glove-side knee
3) Have them throw to their partner, making sure they:
a) rotate their shoulders and point the glove at the target
b) get the throwing elbow above the shoulder
c) turn the wrist so the ball is pointed away from them and the target - getting the wrist rotated at the start of the throw ensures that the wrist is involved in the throw
d) follow through - rotate their shoulders, and "put on the seatbelt" - so that their throwing hand ends up on the outside of the glove-side knee
This enforces key mechanics - wrist, shoulder rotation, follow through
Move the lines further and further apart, get the girls throwing further than they thought they could. Always stress good mechanics, take their time, doing it right is much better than doing it quick - right first, quick will come (the opposite of pitching).
Picture #1: Preparing to throw - notice ball is facing the ground, hips and shoulders oriented to the target. Thrower should be looking at the letters of the player she's throwing to. Where you look is where the ball is going (hopefully).
#2: Notice the elbow getting above the shoulder, and the glove pointed to the target.
#3: hips and shoulders rotating, weight is on the front leg, arm is extended, wrist has come around. Notice the glove is being pulled in toward the body. Stepping into the throw is important, if the throw is consistently high, have them step into the throw more.
#5: Hips fully rotated, wrist has flexed through the throwing notion - as practiced in the wrist drill. If they are doing this correctly there will be a lot of spin on the ball. Notice the throwing shoulder is dipping down and toward the glove-side hip. Notice the glove is being pulled in toward the chest.
#6: Seat belt is clicked in, back foot comes up, shoulders and hips are fully rotated. Notice the glove is pulled completely in to the chest, "pinkie to heart".
Ideally, the ball is gripped with the thumb and two or three fingers. With little hands this is difficult, but make sure they aren't palming the ball - it should be held with the fingers up above the palm even if by a fraction.
Most girls (boys too) leave the glove hand dangling, without really know what it's doing. Bringing the glove hand into the chest or armpit increases the speed of the rotation of the shoulders and increases accuracy (watch MLB pitchers and outfielders). But, it feels strange and is unnatural to begin with, so you probably don't want to stress too much. Athletically inclined kids seem to "get it" and adjust best.
You can read the full article here.