SafetyNet could be made up of willing masternodes that have sophisticated child locks that can't be tampered with by the child.
Nothing is childproof.

More seriously, I am absolutely against any kind of content filtering on both technical, legal and moral grounds. And that's a lot of boths!
Techinical: everything should be encrypted, the system shouldn't be able to determine the content anyway.
Legal: when you allow some things but not others (or try to) you lose your common carrier status and become legally responsible for said content.
Moral:
fuck censorship.The SafetyNet nodes wouldn't be able to see content. SafetyNet could make it known the content it blocks from the off. No need to comb the content.
But yeh i agree with what you said.