There's a fine line between corporal punishment and abuse. Who is to say when it is justified to use violence as a punishment?
Beyond that ruling through fear by inflicting pain on those who aren't yet able to defend themselves is morally wrong on several levels. The first part using fear as a control method is wrong because it doesn't help create a healthy minded individual and because it goes against human rights. The second part is just bullying, it doesn't matter what "cause" it is for it is picking on those weaker than yourself to impose your will. This applies to both parents and authority figures.
IF it were acceptable I see little reason for gender to play a part, man or woman it doesn't matter to a child getting beaten, pain is pain.
When would it stop being acceptable or effective? That is the simplest for me to answer really. A method is no longer acceptable when it doesn't get results. So when the child is old enough to stand up for themselves and fight back it is no longer effective and, if you ask me, no longer appropriate. This is assuming you could even consider it as appropriate to harm a child anyway.