Report your suspicions to law enforcement or to DFCS 1-855 GA CHILD (1-855-422-4453). If reporting in good faith, you are protected from civil and criminal liability.
Even if the investigation fails to prove allegations of abuse, you have helped. Child Protective Services is now aware of the family and its potential for abuse, the parents realize that someone is paying attention to the physical and emotional condition of their child, and the child knows that someone cares.