Dogs and children are two different beings who share one very important trait: they are highly vulnerable and are at the mercy of the decisions made by the adults around them. When you use physical force to discipline either a dog or a child, you're taking full advantage of that vulnerability and are using fear and pain in the place of truly teaching a lesson.
Aggression begets aggression
Force and fear have no place when you're working with innocent, vulnerable beings that soak up our every word and action. Aggression breeds aggression, regardless of species. If you hit your child, (and yes, personally, I do believe that spanking is hitting) you're asking for a child that will likely view physical force as a viable solution in various situations in the future. If you hit your dog, you break the bond of trust and increase the chances that your dog will show aggression or snap much more quickly next time.
No reasonable person would say that dogs and children are one and the same, but research has shown that dogs have the same learning and thinking capabilities as the average toddler. Why not take advantage of the incredible intellect of both children and dogs and use it to teach them exactly what we want and don't want?
Building trust
I am a mother and have raised my 9-year-old daughter to be kind, thoughtful, and respectful. Not once have I ever laid a hand on her. And through the bond of trust we were able to build she has learned appropriate behavior and how to find workable solutions, while I have cemented my belief that it's most valuable to teach with patience and love. Hitting a dog or a child is easy--all it takes is a hand and the willpower to do it. But what's much harder is putting away the paddle and using a much more powerful tool--your mind.
Read a powerful narrative about spanking. (WARNING: Contains Expletives)