I don't think the line is at doctors reporting, and I'll tell you why.
There are many cases of MD's having patients where they know the whole family and can't believe that abuse would be going on, so they don't report things like spiral fractures and pattern bruising in a five year old. Those are markers of serious domestic violence and abuse, but since the doc knows the parent, and the parent has a sorta reasonable explanation, fine.
No. The doctor reports, then the parents get investigated. Sorry it sucks, but the point to fix is the people interacting with the family at the point of investigation, not the report by an MD, because unfortunately the MD is going to lean towards not reporting until it is too late.