Do Unto Others

My two girls and I were traveling on an air plane recently. Amidst the crazy dealings, the 5 year old proceeded to put her bare foot into the 2 year old's face. I (as any good mother would) said, "Remember, do unto others as you would have them do to you." There, that should do it. Now there will be peace...Yeah right. She quickly responded, "Okay, I'll put my foot in my face and then I can put it in hers." So where do I go from there?