ITA. You're either part of the solution or part of the problem. If you know or suspect abuse and do nothing then you are enabling it. You are telling the adults that it is ok, and the children that they deserve it. The number 1 cause of death of children is homicide by parents, in almost every case at least 1 person knew knew and did nothing. Those who do survive often suffer long term psychological illness. All it takes is 1 phone call.
But all that aside, if the children are truly living in the kind of situation you describe, the authorities should be called. To me, knowing these children are abused by a mother who throws them around and does drugs and doing nothing about that is the truly terrible thing that is happening.