As I understand it, if he marks his videos as being for kids, they end up effectively demonetized and comments aren't allowed. If he doesn't mark them as being for kids, he's at risk of massive fines. If he marks them as adults-only (or just starts swearing or the like), they're also effectively demonetized.
Guess this explains all the push by a lot of creators towards Patreon and the like - the simple solution is to have your videos on YouTube marked as for kids, but to push for people to support you on Patreon to get access to discussions, a channel-specific Discord, etc. for the kinds of things that would previously be in comments.
Edit: Oh, and no notifications when new kid-friendly content is posted, so if you want notifications 'Support me on Patreon'