I agree with Lenski. I think the absolute worst thing a coach or parent can yell to a hitter-- besides, "You stink. You'll never get a hit!" -- is "watch for the change up!" If you're looking for the change up and don't get it, you might as well sit down because any other pitch is going right by you.
That being said, it's perfectly all right to go up to the plate and sit on the change up early in the count if the pitcher has been throwing a lot of them. Just stand there with your bat on your shoulder and wait until the ball is on the way before you lift your hands. If the change up comes, you'll have time to lift, turn and hit it out of sight! If anything else comes, you won't have time to do anything but let it go. I think it's something every coach should help his/her team to learn to do. Every once in awhile, especially at the 12 and 14U levels, you'll face somebody who is either really slow or who throws the change about every third pitch. If this little speech, "all right girls, let's go up there this inning and sit on the change up just the way we practiced," is in your arsenal, you've got a good chance to knock her out of the circle in a hurry.
But when your hitters have two strikes, DON'T YELL "WATCH FOR THE CHANGE UP!" Say, "Keep your hands back and put the ball in play," instead.