By Horav Yosef Yeshaya Braun, member of the Badatz of Crown Heights
The prohibition of mesirah, informing secular authorities about the behavior of another Jew, is quite severe and constitutes a grave offense.
Nonetheless, if a person is aware that someone is, G-d forbid, endangering the welfare of a member of society, according to halachah there is absolutely no prohibition of mesirah; it is permissible to inform the authorities about that individual’s behavior. The details of this halachah can be found in Shulchan Aruch Choshen Mishpat, Siman 388.
The consensus of poskim today is that if someone has a reasonable suspicion that a child, or children, are being, or have been abused (defined as any action or failure to act that can cause serious emotional or physical harm), they are not only permitted to report the suspect to the authorities, but indeed should do so without delay in order to protect these vulnerable children from abuse.