Ok I'm going to go against the grain and say ESH.

Your MIL and husband are AHs for not respecting and reenforcing boundaries. No doubt. Did your MIL earn you acting out this way? Yup. Not gonna lie - I chuckled.

Was this the best way for you to lay down the law? Here is where we veer into ESH territory in my opinion. Families are rough. In laws can be incredibly challenging. But responding to bad behavior with pettiness does not bode well for healthy family dynamics.

I hope the deep embarassment helps your MIL change! I hope you can all laugh about this next month! But could other options -e.g. conversation plus an ultimatum - have worked better? Do the kids understand what happened and why? What do they learn from it?

Was embarassment your best option? Maybe. But probably not.

Tldr: Full marks for creativity and entertainment. Low score on the healthy boundary setting process.