I agree. Saying "mind your P's and Q's" is a little hoity toity. I picture disconnected, workaholic parents saying this to their kids before they go to Sunday Brunch at their Country Club dressed in little suit coats and dresses.

Many people would say this to their kids instead, "This is a special event and everyone is going to be using very good manners. There will be no fighting, screaming, or annoying your siblings. If any of you act up, and I mean even a little, I'm taking your tablets away for a MONTH! I mean it. Don't test me on this!"