I've written several stories where perfect strangers say things like that (and sometimes deliver), but we have moved beyond the point where adults judge the behaviour of other people's kids.
That said, a couple of years ago when I was out walking (exercise is a good thing) I heard a mother calling to her kid, saying something about getting in the house now or someone would have a red bum. The kid in question was with a couple of friends and I saw him roll his eyes at his friends, a clear "she's joking" sign, but when she call "I meant now" the kid did go inside the house.
Nope, I know nothing about the family nor have I heard that threat again, but that might be because I vary my walks so
A) they don't get boring, and
B) people don't notice a stranger walking down residential streets and wonder what I'm doing there.
Here's a quick link to a comic that's vaguely on that topic. It's someone in retail making an oblique reference, something that would probably get him fired in real life.
http://retailcomic.com/comics/august-31-2009/Note: as far as I know that's the only spanking linked bit in that webcomic. Most other interactions with unruly kids are of the "how dare you tell my kid to stop doing something dangerous" variety.
Goodgulf