Our dc finish their main course, say thank you and ask to leave the table. If there's pudding they may play until its ready. Our friends get their children to wait until everyone's finished. I think that's fair enough when everyone is eating sensibly and at a reasonable pace. But if it's a meal where adults are taking a long time and chatting, or (as has prompted this post) its a children's meal where my children have eaten sensibly and the other two are messing about and being cajoled into eating it doesn't seem fair. When mine get down, they go to a different room and don't disturb the others still eating. I know that when they're bigger the expectation will be to stay til everyone's done, but I think waiting up to twenty minutes with very little to occupy them except watching their friends (not) eating seems to teach very little.