I've dealt with CPS before and they come to you with a "safety plan". They typically present it as if you have no choice (sign this agreement saying dad will move out and not be around the children or we will send the kids to foster care). In reality though, if you don't agree to it, then they would have to get a court order from a judge before they could really enforce anything (CPS alone does not have the power to remove the children, except maybe temporarily for a couple days in some states).
If the parents don't know any better and just agree to the safety plan - NO PROOF is required for this. CPS could be acting on a tip or a hunch or anything without any evidence to back it up.
As far as getting charged, that takes more time and more proof. CPS typically acts first and then builds their case. The police typically build their case and then act. Once CPS has finished their investigation, they will then turn all of their notes over to the DA who then decides if there is enough evidence for the police to open and investigation and charge him with something.