I don't think children under the age of 3 should be smacked, spanked, or otherwise hit. But I don't think a law is going to do much to prevent it. Nor will it prevent shaken baby syndrome, babies being dangled from balconies or dropped on their heads, or small children being yanked by the arm (it happened to me when I was about 3, and my shoulder is still out of whack!). Laws don't really have a great track record in preventing inappropriate behavior. Just ask Moses...or God.
I am personally opposed to spanking or any other form of corporal punishment. Besides having the possibility of causing serious injury, I think it gives the child bad example. It makes it "okay" to hit another person when they fail to meet your expectations.
But I seriously dispute the wisdom of making a law prohibiting parents from ever striking their own children. Make no mistake about it, if this law is enacted, the age will be raised, and it will become a crime to strike children right through their teen years. Two- and three-year-olds are not likely to call the cops when Daddy smacks them. Teenagers have 911-capable cell phones glued to their ears.
When I was in high school, one of my teachers used to mock the mentality that says "There ought to be a law" whenever anything in the world seemed wrong. The more laws you have, the more laws get broken; and some laws are made to be broken.
Sometimes a quick slap on the rear or across the face is the quickest and most effective way of telling a child that a given act or word is unacceptable. Used as an outlet for parental frustration and anger, a spanking or even just a slap can be counterproductive. Used as an instrument of correction and teaching, it can be worth thousands of words. A child knows the difference.
Having experienced spankings myself, by hand, strap, and hairbrush, I am strongly against that type of spanking. Spanking to hurt, beating a child to "within an inch of your life", raising welts, etc., I believe to be wrong. All they breed in the child is anger, resentment, and the desire to get even. I know that my experiences didn't teach me anything about whatever infraction I had allegedly committed. They did build up quite a wall between me an my father.
There needs to be a line over which the parent must not step. There does not need to be a total prohibition. I don't think a law will do it. We already have laws governing brutality and abuse. Maybe some of them need to be refined. Education would be helpful: what is appropriate and what is not appropriate for child rearing might be incorporated into the overall educational process in the schools and in the home.
The best teaching, of course, is by example. Parents are the first and best teachers of their children in such matters--and I would include grandparents here as well. My wife's paternal grandmother raised nine children, eight of them boys. As the story goes, she never hit any one of them even once. Her children and her grandchildren were not always well-behaved, but all it took was a word or a look and they snapped right back into line.
To sum up my feelings on the matter, spankings done for the sake of retaliation or only to punish are wrong and should be avoided. A quick slap or two administered to call quick attention and admonish a child, as long as it done in a way not likely to cause real physical injury, is perfectly acceptable as a parental teaching tool. There should not be a law.