If you forced an adult to eat, or to go to certain places or to do a number of things parents typically make kids do you would be charged too. Children arent adults. This argument was always dumb and always will be
You are in custody of the child though. They have to eat in order to live and survive. If you are the caretaker for an elderly person it is the same way. You take certain responsibilities. I don't think one of those responsibilities is hitting the person you are caring for.
I understand that children need to be taught things by their parents, but there is no evidence that spanking or hitting actually works. Also, we as a society used to tolerate many forms of abuse to women, children, animals, handicapped people. We told people that wasn't cool and made efforts to curb it.
The only reason spanking kids isn't widely condemned in this country is that we have some feeling that we get to decide to do what we want with our "property." It's still cool to think that children are property and not a living creature/person worthy of respect.