Here is a PDF with Teshuvos (responsa) from various poskim, including Rav Elyashiv, on halachically "mandated reporting".
Your concerns about Loshon Hora and Mesira are justified; that is why Rav Elyashiv is here to tell you that you have nothing to fear about reporting child molesters.
In short, you may report them to the authorities.
Does he say that you must? I did not read the entire thing; find out for yourself.
But don't hide behind "It's an aveirah!" It's more of an aveirah to allow children to be spiritually and emotionally murdered. OK?
NEW(added April 5, 2011): Here are some more PDFs.
Yeshurun 15 Shaarei Halacha Pamphlet
Yeshurun on Reporting Abuse
Rav Elyashiv's Tshuva (responsum)
Translation of Rav Elyashiv's Tshuva
P.S. The PDFs were kindly provided by Dr. Asher Lipner, a frum psychologist at the vanguard of the war on abuse in our community.