Yes, absolutely this. Reporting to the FBI via their "anonymous" form is also a no go. Both can and will ignore your good intentions and just nail you outright.
4chan has seen bots spamming CP links for a while on the titty boards; Twitter and Tumblr both have Arabic users openly posting CP (because telling them to stop raping children or act even remotely civilized would be ISLAMOPHOBIC); And hell, Tumblr had goddamned supremely exceptional users who were literally creating their own CP and posting it because "I'm a minor and laws don't apply to me because I don't wanna obey them so I don't have to, tee hee."
Your best bet if something like this occurs? Don't tell the goddamned cops or @ the FBI or what have you, that'll just get you Vanned and everything in your house that can store data siezed for 5 years. Close your browser, use CCLeaner to secure wipe your files (3 passes), wipe the free space using CCleaner's advanced options, and then use Eraser to 3 pass secure wipe the browser's appdata folder entirely.
(Oh, and don't fucking livestream it like the Killstream exceptional individuals.)