What liability does a person bear if he knows of child abuse but does not report it?

Some people are what are called "mandated reporters." The law requires them to report known or suspected child abuse. Teachers, doctors, nurses, therapists and so forth. There is a statute which lists them. Someone is not a mandated reporter unless he or she knows it, and he or she knows it because he or she learned of mandated reporter status as part of professional training.

A lawyer CANNOT report his client's past crime because of the attorney-client privilege, which is sacrosanct.

Hope this helps. It's not legal advice, as I don't hold California licensure. It's simply my analysis of your question in light of general principles of law, intended to educate. If you need legal advice, please consult a lawyer who holds California licensure.