The spanking - short film -mkup81

The 9 year old Max offers his mother Jody the instruction book for a perfect spanking. He explains to her that a good smack on the bottom is effective for the education of a child. Together Max and Jody put into practice the manual.
Why do we call "crualty" when someone is hurting an animal, "violence" when hurting an adulte and "education" when hurting a child?