From Revach.net w/ permission:
If a child is brought to a hospital battered and showing signs of cruel abuse
such as burns, broken bones, black and blue marks it is forbidden for a Jewish
doctor to return the child to his parents custody and he must immediately report
it to the authorities who will remove the child from the home and place him in
foster care. The Doctor does not have a Din of a Moseir since returning the
child is Sakanas Nefashos.
What if the child may ultimately end up in a
non-Jewish home or institution? The Nishmas Avrohom (CM 388:1) says that Rav
Shlomo Zalman Auerbach and ybc"l Rav Elyashiv both hold that you must still
report the abuse to the authorities, but you must do everything possible to try
to make sure the child ends up in a home that is Shomer Torah U'Mitzvos. He
brings from the Tzitz Eliezer that the doctor is only removing the child from
his parents murderous arms. What the government does after that is not his
immediate concern.
Furthermore he writes that Rav Shlomo Zalman and the
Tzitz Eliezer both explicitly held that this halacha applies when a father
sexually abuses his daughter, even if she is a ketana, since the halacha equates
saving a girl from this abuse to saving her from murder.
עד כאן
See also the journal Yeshurun [Volume 15] where there are a collection of articles on this critical topic and of course echo the above-mentioned psak.