The key to teaching how to throw is to do it backwards.
Teach the end of the motion first, then add the earlier steps one at a time, working your way back to the start.
The grip. Your child should grip the ball with three fingers over the top, and the thumb underneath. The ball is held with the fingers — it shouldn’t touch the palm.
The stance. Have your child stand with his feet wider than shoulder-400 apart, facing sideways to the direction he’s throwing, with his throwing arm furthest from the target. Have him turn at the waist so that his upper body faces the target, and have him hold the ball so his arm looks like the letter L. His elbow is at shoulder height and his hand is pointing straight up.
Next, have him lay his wrist back, then snap it forward, straightening the elbow and releasing the ball. His arm and fingers should finish pointing directly at the target. The ball won’t go very far, but it should go in a straight line.
The windup. Get your junior pitcher into the same stance as before. He should raise his throwing arm again to make another L. Have him turn toward the target, keeping his elbow high and his hand pointing straight up. As he finishes his turn, he should snap his arm and wrist forward, pointing at the target on the follow-through.
The follow-through. Have him step directly toward the target with his front foot, and push off with his back foot. As his weight moves forward, he should turn his body and follow through. When he finishes, all his weight should be on the front leg, and that knee should be bent. His arm and fingers should point at the target.
Do this a bunch of times, going slowly until the motion looks natural and smooth. The sequence of movements is important: step, then turn as the weight comes forward, then snap the wrist into the follow-through.