[fnordfnordfnord]

You say that as if the cost of helping is near zero. What if authorities decided that your hobby project's server was interesting to their investigation, and subsequently showed up at your home with a warrant to seize every electronic device in your home/business, including the server that hosts your business, as well as unrelated things like cell phones, video game consoles, etc?

[DasIch]

So instead of going to the authorities directly, describing the situation and offering to work together you propose sticking your head in the sand, trying to deal with the problem on your own and hoping the authorities won't ever come across child pornography on your site?

Especially at a scale where you need automated systems to deal with the problem, law enforcement will inevitably notice sooner or later. I can't help but fell that it's not going to go over well with them (and it shouldn't), if they notice you deleted that content and possibly destroyed evidence in the process.

Technology companies and law enforcement have cooperated on this issue for a long time very successfully. They have experienced people working on nothing but this kind of thing and you're not going to deal with some local low level idiot that barely manages to deal with noise complaints. There is no reason to be paranoid and to believe they are going to act stupid.