I have always gone by Bertrand Russell's rule: "never strike a child except in anger". Children can understand a lash out when someone has been pushed too far, it's an expression of feelings and it passes along with the anger. It's calculated punishments that teach them that violence is acceptable.

We obvously have a range of opinions on childrearing here just as anywhere else, but I'd hope that practicing BDSMers have their issues with pain and violence sufficiently worked out that they can keep them separate from parenting. It's people who haven't accepted their own feelings who are likely to beat a child and call it punishment because they can't admit to themselves that they enjoy beating people.