How much much should you provide to the kids before you start charging extra for milk?

I have one kid his parents say to give a bottle when he goes down for a nap, so I do what I am asked. I provide 8 ounce milk to him, and if he doesn't go down the want me to give him 8 to 12 more ounce of milk meaning 2 more bottles. That is more than half a bag of milk I just have given to the child.