I agree with annsixty , we reserve smacks for life threatening situations, running in the road , sticking that darn fork in the socket!!
In the 9 years we've had grandchildren we've used it once ..... For a running away in a car park, and one stern 'no' from Grampy for felt pen drawing on the table. Both were our eldest DGD now aged 9.
We don't smack, not because I don't believe in it - in the right place a small smack on the hand or on a well padded bottom might be necessary- but because we are consistent with our house rules.
There aren't many, they never change and we never threaten. The children 9,6,4 and 2 all know them . I watch their parents say things like ' if you do that again' for the 53rd time and I chuckle . The reason it's being said for the 53rd time is because they aren't consistent . They just want the path of least resistance. And I get that. It's exhausting raising children who constantly push the boundaries.
In our house, it's our rules. I gave up waiting for my DDs and SILs to discipline their children so now if a child misbehaves there's no threats , it's off to the naughty step where they stay till I fetch them ( unlike in their own home where the little ones make the rules ).
My DDs know how it works and at any time they could step in and do the necessary but that would mean getting up, stopping chatting or putting the phone down.