On the Child Safety page one of the dialogs is the opt in (or out) configuration, so it seems, as one would expect, that the adult(s) in the family sharing group get to configure this.

And it's a useful, valuable option that many (I would wager the overwhelming majority) parents will enable.

Apple made a huge PR mistake announcing both of these systems together (the CP hashing system and the NN message warning system), because as seen throughout the comments it has led to lots of people conflating and mixing and matching elements of both into a fearsome frankensystems.