I constantly have a stream of guests for tea. I have a large family and (popular?)dc always being asked/asking to have friends back, and I am very welcoming, lots of freshly homemade food (sometimes!) baked beans and sausages etc (when I'm too busy or can't be arsed to make more of an effort.
Anyway, what I do insist on is the dc say please and thank you and if they forget I remind them (ages range from 3-10).(politely of course)
I recently had parent here whose dc I brought back from school and they all stayed on for tea, I reminded their ds (aged 7) to say p and t in front of parent and thereafter they prompted dc.