. Website refrenced below...
I've got a 9 year old as well. There's an app called up my game. You can upload a video and for $10 solicit an "expert". The baseball guy is former MLB and seems OK.
I review on there as a swimming coach and it's just starting up still small. Maybe worth $10 to see if it works for you guys.
For me, I've decided to take it on for my boys until they pass my ability to research and help. Not there yet. So much data out there and I studied my tail off for several months understanding the basics. I've gotten a lot of good advice reading on here but I also have 25 years coaching and video reviews for different sport but transferable skill so far.
What's been working for us is I video his motion, digest it, we discuss it, identity ONE key point and follow up with a drill to reinforce it. One key point at a time. Then repeat that drill every time we throw as part of our routine until he doesn't need to think about it anymore.
I can tell when I create a little imaginationary pressure to hurry him a bit or distract his mind by forcing his focus on something away from the movement and it's still in good shape on video. Then I remove the drill and go to the next one. Only keep 3 active at once.