@Flick Responding to your question on Vivaldi Social, it's somewhat sanitised but wouldn't say 'kid-safe' as it federates with instances hosting some of the types you wouldn't want him to see or interact with. I like Vivaldi (the browser) for other reasons though.

This may require upfront effort finding someone who can spin up a single-user instance. Mastodon has made this easier but has also made it harder due to said people coming from Twitter/Tumblr/etc. having some impact on the micro-communities each instance fostered.

I'm afraid I have no discrete suggestions. They're tainted one way or another.