It might be difficult for a 9 year old but there was an interesting series on YouTube or twitch called handmade hero. I didn’t follow it all the way through but

It might be difficult for a 9 year old but there was an interesting series on YouTube or twitch called handmade hero. I didn’t follow it all the way through but the person hosting it did a weekly livestream and just started writing a game and engine from scratch in c. I don’t know if it’s still active but there are 600 videos in a YouTube playlist. Even if not sticking with games I think it’s possible to learn a lot from the series.