After a couple of threads on here on the subject (and I realise I'm starting another one!) I asked DS1 about it the other day, calmly. He's 3.10.
I asked him how he'd feel if smacking him was something DH and I did (and reassured him we weren't about to!) as a punishment.
He said, and I quote, "That would make me very sad, Mummy, and it would NOT be very kind at all!"
Have you asked your children how they feel about it, whether you smack or not?