I have this with my 2.6 yr old, who openly thinks it's hilarious when I go in to explain why he's been placed in his cot for the last few minutes because of some misdemeanor, but also with my 4.9 year old, who actually tries to hide his smile with his hand on occasion (also does this with his teacher at school...).
I find it SO frustrating.
I am not soft - in fact, I think I over-do it and seem to have a go so often that the DCs know they get the same (exaggerated, perhaps) reaction whether they've, for example, splashed water all over the bathroom (mild violation in my normal, reasonable frame-of-parenting-mind) or hit sibling/dog/Mum/other child (serious violation).
It very much feels as though there's a 'whatever' attitude in this house and do I just need to chill out?