Eight years ago, I reported a mother to CPS. She would direct her drunk boyfriend to beat her son with a belt because she couldn't catch him.
CPS gave me the usual "We'll investigate."
After I found that they weren't doing much, I called CPS again & said, "I'm about to write editorials to several newspapers describing your apathy & incompetence in detail - including names of everyone I spoke to in your office. I'm also going to write to several TV news programs about how you're paid to do nothing. Then, when the child is found dead (or he kills his mother,) how do you think you will look?"
CPS sent a social worker to the child's school to interview him, then they forced his mother to attend parenting classes & anger management, or lose custody. They also suggested she not allow her boyfriend in her house when the child was there. I kept in touch with her; she did comply with both requests. CPS did not tell her who reported her.
I occasionally talk with the son; he's now 20 & living out of state.