Professor Robert Weisberg is being quoted in this article about a California bill that might make spanking a crime:
Robert Weisberg, a professor of law at Stanford and director of the Stanford Criminal Justice Center, agreed. "Look, you can make a constitutional argument out of anything," he says, "but the broadest argument you could make here -- that there is a core of family privacy that can't be violated -- won't work. There is an old Supreme Court doctrine that says there is some core of family autonomy which the court can't touch. But that mostly applies to religion and education, not discipline."
Yet a no-spanking law may not make much of a difference. Weisberg says most states have laws against assault that contain exceptions for parental discipline. "Spanking a child is assault, but these exceptions permit some battery against a child, if you define battery as unwanted, harsh physical contact. So would an anti-spanking law make anything different?" he asks. "It probably won't advance things beyond the situation that exists today."