The Debate Over the State
As to the state prohibiting spanking, there is no doubt but that once the state has established such a law, along with the bureaucracy needed to enforce it, that said bureaucracy will seek to expand itself, as all bureaucracies do, by seeking through legislation (or, as most on the left are wont, through judicial decree) to outlaw other forms of discipline as well, until, eventually, the state will sit as mediator of the parent/child relationship. This will destroy parental prerogative in discipline and, by extension, the traditional family, thus accomplishing Karl Marx's dream.
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If there is a separation of church and state, there should be a separation of state and parent/child relationships unless the relationship is unhealthy and is causing harm to either the parent or child.
As for, "bureaucracy will seek to expand itself, as all bureaucracies do", I agree. Those who are given something, want more and more of it. If the state can already control activities we do or don't do at work and school, why should they have control of what we do at home? If the state is controlling the method of discipline a parent is using with their child then the state and not the parent is controlling the development of that individual. Even though government interference is not a bad thing for some families since the parents are unfit to take care of their children in a safe manner, those parents probably shouldn't have had kids in the first place.
Rosemond commits what is known as "The Slippery Slope Fallacy", by assuming that if the state forbids hitting children, then it will pass more regulations until it has overwhelming control. By this reasoning, if the state continues to forbid hitting your neighbor, then it will pass more regulations until it has overwhelming control.