Deliberately physically harming a person in any way, most of all a child, should without a doubt be punishable by law. However, the words "hit", "smack" and "spank" should not be used interchangeably. To hit somebody is to forcefully strike them, usually to cause pain, whereas a spank is merely a remonstrative tap. I would never hit an animal, and would despise anybody who did, but giving my cat a painless tap on the nose to teach her not to steal food is completely different. Similarly, it's natural and very healthy for a parent to have a strong physical bond with their child and to communicate with them through their body, and this can include the occasional painless slap to establish a safety boundary, particularly with toddlers who need to understand quickly that certain things are dangerous. If that's aggression, telling someone off is verbal abuse.
The phrasing "unreasonable force" is definitely ambiguous and should be ammended, but spanking shouldn't be outlawed altogether.