May I Please Be Excused

Ok. So I like my children to be polite. And well behaved. And have good manners.
So everytime they finish a meal-meaning they have cleared their plates and want no more additional servings they have to ask the adult at the table "May I please be excused?" and only after being given permisssion may they get up and remove their cup and plate from the table.
The girls were just finishing up dinner, and Jessie finished first and asked if she could be excused-her plate was clear-so I said yes and she cleared her place and went to her room. Not two minutes later do I hear "may i please be excused" again.....but it's not Nina's voice...I turn t Trin and I go "What did you say?" and she says it AGAIN! I'm quite sure she has no idea what the majority of the words in that statement mean but she has recognized that, collectively, they represent the completion of a meal-b/c her plate was CLEAR.... and I said "are you finished? no more?" and she said "finished" and gave me her plate and spoon.
my big-big girl.