This is an intriguing idea. It certainly would be possible for a client to be developed that was intended to make Nostr more kid friendly by filtering ALL content by default, and allowing parents to whitelist only certain npubs.

Key delegation could be a part of this client, as well, once we have a compelling solution for it. A parent could create an npub for their child that requires a signature from the parent for certain types of events, such as adding another user to the whitelist.

My point is that Nostr, as it currently stands, is not kid friendly, and we should not expect it to be, given the purpose it serves as a censorship resistant communication protocol intended to bolster free speech.

If free speech doesn't include forms of expression we don't like, or would prefer others refrained from in public, then it's not really free, is it?