Disagree with a reason, never with "that's stupid".
Listening without interrupting, letting others finish and respecting different tastes are habits children use far beyond the dinner table.
Kalmdix announces the mission as a playful quest at the start of the meal, then lets the children talk about whatever they like. It works on one habit over five meals; on the fifth, the mission is a secret. Kalmdix never says who did or didn't do it: the results appear after the meal.