In the USA, you need to contact NCMEC - http://www.missingkids.com/home or the FBI.

From: Mark Seiden <mis@seiden.com>
Date: Friday, 7 December 2018 at 12:16 PM
To: Suresh Ramasubramanian <ops.lists@gmail.com>
Cc: "Lotia, Pratik M" <Pratik.Lotia@charter.com>, "nanog@nanog.org" <nanog@nanog.org>
Subject: Re: Should ISP block child pornography?

thanks, suresh. what it seems to say is get in touch with the ncb in your country to sign an nda and get instructions.  (but it's actually quite hard to figure out how to do that, no email address or phone numbers apparent for interpol dc)