I listened to a Coach Woolforth - web radio broadcast last night and he talked about teaching a changeup. He said the pitch is his 2nd favorite pitch. He said his favorite pitch is a “big hairy, well located fastball”.
He also said that throwing a changeup much below 15U & 16U year isn’t necessary since the only thing you are doing is slowing down your fastball & most kids below that age have trouble hitting a good fastball but don’t have trouble hitting a slower pitch, such as a changeup. When you get to the high school level or more elite travel levels you can start throwing the pitch to good aggressive hitters and they may swing at it. He did say to be working on it though because you should be preparing for the next level of baseball. He also said you could ask 99 pitches to show you a grip they use for their changeup and you would get 100 different answers. So grip wise there is no one right grip.
A couple of teaching points that I have learned are:
Make sure the pitcher isn’t slowing down his body or arm speed to take velocity off the ball. Velocity change should come from the grip. I worked this in my pens by alternating pitches between a fastball and changeup. I would throw one fast ball followed by a changeup & do this for the whole pen. If you catch his pens - tell him not to worry about where the changeup goes initially - but pay attention to what his motion & tempo looks like - if he slows it down - tell him about it & keep working on it until he has the same action for both pitches.
To keep my arm speed the same I would use my change up grip on some throws when I was doing long toss. With a 9 year old - it might not be long-toss but just when you are playing catch with him in the backyard.
I do agree with the what other people have said that at 9 - there probably isn’t a reason to throwing a changeup during a game - but I think learning it does make pitching more interesting.
I have worked on a straight change for 3 years now and can finally throw it with good movement and location and I have used it in games - but the first couple of games I threw it in - it got hit pretty good - so something else to remember is not to give up on it.