As long as the slap isnt the equivalent of a bag of bricks hitting the child in the face, i dont see why not. It disciplines them and prepares them mentaly for the adult world. Because lets face it, there is much worse than a smack on the backside. It also teaches them right from wrong, they will eventualy realise: 'Swearing at daddy brings pain' and 'Being nice to daddy doesnt bring pain', so realisticly there going to go with no pain.
Thats just my opinion.

Haha yes, pain is a effective way to teach but also not the best.
As you said though, it's not like hitting a child with a bag of bricks haha

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There is a difference between discipline and down right abuse. Though I still don't think pain is the best way to show your child good or bad. There are so many other ways...Better ways.