Eldest twice. Once for repeatedly trying to run into roads when all efforts to explain and reason with him failed and once when he was 12 and decided to show off in front of his new mate, thinking he was big and hard enough to disrespect me. Swiftly availed him of that notion.
Second child once for kicking the dog during a tantrum; she didn't try that twice.
Third child once, shortly before the Peppa pig ban, when she decided to copy her and talk to me like absolute shit. Because that pig is fucking rude. Sharp attitude adjustment and no more Peppa.
Fourth child, never needed to. She is young and autistic and for the most part, smacking her would do no good, she isn't being defiant, she doesn't understand. As she gets older, she will be held to the same standards, I wouldn't let her disrespect me but you do have to make allowances for her level of understanding.