I would say:
I plan to give the moon cake for someone and tell her how much it cost. Show that you are not happy, because you have to buy again. Tell her that next time, pls don't eat the special thing apart from 3 meals.
At the beginning, I will give her something special to eat, e.g. cake, fruits. This will make her a habit that if I don't give her to eat, she won't eat without my permission.
I understand your feeling. Because I have a baby too, if I blame her too much, I am afraid she would do anything bad to our babies.... :-(