US Customs apparently has authority.
http://www.customs.ustreas.gov/enforcem/child.htm
This might be useful
http://www.prevent-abuse-now.com/law3a.htm#Report
http://www.spamcon.org/recipients/spam-response/report-illegal.shtml
Some spammers will be as provocative as possible to pull traffic. Site may be "normal" ****.
Some try to get other sites/people in trouble.
Some will post a phone number to bait you to call even if to complain. These can redirect you out of the country to act like a 900 number and suck some large money out of you.
You don't necessarily have a real kiddie **** person. I wouldn't think they normally try to get too much attention. But who knows. Even if not, though, it's pretty sleazy.
Also, sometimes gov does stings to see what freaks respond.
US Customs