BlockTube also has an advanced blocking feature[0], which lets you define blocks using JavaScript if statements. You can flip it around to a whitelist-system by returning true (block) by default, and returning false (don't block) on your rules. But this does hide videos from search, recommended, homepage, etc.

What you're suggesting could potentially be done by creating an extension which checks the channel_id, then blocks calls to the /videoplayback endpoint if the channel is not whitelisted. Though do be aware that titles and thumbnails could also be explicit.

Side note: All of this is very technical and not suitable for the average parent, which is not great.

[0] https://github.com/amitbl/blocktube/wiki/Advanced-Blocking