For our kid, we ended up installing youtube vanced on his device after he hated the vapid wasteland that is youtube kids (he'd used youtube on computers before, so he knew what he was missing). To be honest, I almost think (not seriously!) a filter which disallowed content that is allowed on youtube kids would be a more fulfilling experience - get rid of the hellscape.
To be fair, your mileage will absolutely vary - I assume most nine year olds don't like deep dives into electronics, phone tear downs, and people building retro lego sets, and also of course this does mean some supervision is required - but then, at that age, when isn't it? This works for us, might not work for you, etc. But, our experience is the current youtube kids is a worst-of-all-worlds compromise, and it's frustrating that google won't let us just say "actually, no, just let him use real youtube plz".
(We do also pay for premium youtube, so he doesn't see ads, which makes me somewhat more comfortable - again, though, supervision etc)