I don't think it should be illegal. Lots of very well adjusted kids have come out of both kinds of homes - those that hit and those that don't. And lots of very maladjusted kids have come out of both kinds homes. I would be slow to let the law micromanage what parents can and can't do with their children. There seems to be a consensus among reasonable people about what constitutes abuse. In the U.S., most people don't seem to thing that a smack is automatically abuse. I don't think you can enforce a law that is not in line with what most people think is fair.

In modern times, it seems that there is a trend away from smacking kids, which I think is a good thing. But I would hesitate to say that it is never appropriate.

It seems like America is evolving in the direction of people separating into two camps - those who are disinclined to smack at all and those who are excessively inclined to use physical force on their children. I'm not sure what society can do about the latter group.

I see, after a bit of research on the web, that there are many European countries that have made smacking totally illegal. A link is below. See the map. I wonder how that is working out.

Corporal punishment - Wikipedia, the free encyclopedia

I don't think anyone other than parents (or whoever is the legal guardian) should smack a child.