Is verbal or mental abuse considered Child Abuse? If I am a witness, do I have to legally report it?

Child Abuse is generally considered as physical and sexual abuse, but also includes child negligence where improper care causing harm to the Child's well being is exhibited. Each State has their own statutes and laws surrounding Child Abuse. Some states consider mental/verbal abuse as forms of Child abuse, while others primarily consider physical/sexual abuse as a violation.

Please find your state on this page which includes Child Abuse State Statutes.

You should not hesitate on contacting the proper authorities regarding this particular situation. Not only are the parents/guardians held fully liable and punishable for committing Child Abuse, but related parties who have clearly observed the abuse and do not report it are held liable as "failure to report." It's basically along the same lines asthe concept of seeing someone you know committ a murder/serious crime, and not telling informing the police about it.

The typical conviction for failure to report Child Abuse is a misdemeanor for first offense, while the actual guardians committing the abuse are facing either a Misdemeanor or Felony among other punishments depending on the severity of the abuse case.