I've also been pondering this. 

I'm generally opposed to handing kids smartphones, at least until they're teens. But at that point you could use parental controls to whitelist clients that have built in features that prevent certain content (i.e. "teen friendly", whatever that is..). Challenge with nostr is its benefit; openness. There are so many ways to access the protocol and find nsfw content..

Thinking out loud: What about a solution where you are the father-npub, and for the son/daughter-npub to see any content it needs to be approved by you (e.g. followed by you or those you follow). Of course, they could spin up their own npubs and work their way around that pretty quickly when they understand how nostr works.