kfb,
This is not not true, unless you are a mandatory reporter such as police officer or maybe a teacher you will not be prosecuted for not reporting. however most other people are not mandatory reporters.
The reason one should consult with COMPETENT Rav, is because calling ACS can ruin a family and their life forever. Therefore; before doing such a drastic move which can have drastic effects you should consult with a Rav that has knowledge of abuse and what the implications of abuse and calling ACS are.