If you do wish to report the site to a US based organisation you can do so by going here…
web.cybertip.org/cyberTipII.html
If you do report it to cybertip it would probably still be worthwhile reporting it to the IWF as well.
It might be best not to e-mail abuse to NAMBLA directly as, personally, I don’t like the idea of a pissed off child molestor having my e-mail address because he could, in theory at least, subscribe you to some “unplesant” mailing lists.