Everyone wants to dance around the subject, but you can't have your cake and eat it too. True anonymity makes the criminalization of the transfer of child pornography unenforceable.
Making bad laws unenforceable is the point of anonymity. But you've got to take the end of good laws along with the bad.
The same way we've been catching criminals for years. Surveillance, warrants, legal due process, etc.
I really don't feel that some reasonable tools in the hands of law enforcement but also checked by the justice system need to encroach on anonymity, TOR or any existing technology. Because despite these technologies, it's remarkably easy for a person to get caught if law enforcement is determined to catch him if they use the same tools they’ve used for generations.