I think children should eat their own lunch, if they are required to bring it. If the school supplies lunch, then they could all have the lunch, but from their own plates and with their own spoons. Mothers would know what and how much to pack for their own child. Mixing them up is something I would not advocate. But you will have to discuss the matter tactfully with the teacher and arrive at some understanding. I agree with you that sharing can and will come later.