When do you start to tell children "Santa is watching"?

purplepoppy
Your child's being a little shit as usual. Normally as a punishment, you'd throw a cherished soft toy to the dog and make them watch as it's ripped apart. Perhaps you'd send them to bed telling them if they open their eyes the monster will suck out their eyeballs or the classic make them stand in the garden until the boogy man comes to collect them.

But with Christmas around the corner, it's time to squash non-conformance with "Santa's watching." No kid wants to be on Santa's naughty look list, but when do you start using that threat?
When do you start to tell children Santa is watching?
When do you start to tell children "Santa is watching"?
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