i would just write a friendly note, requesting that this doesn't happen again, and explaining why. perhaps you could suggest that the nursery have some spare food, e.g cartons of drink etc, in case this happens, and request a larger discount on next month's bill as they failed to care for your daughter properly.
it seems very odd tbh, as it would just have been more logical if every kid gave a bit to J, or the staff divided up the food, or even gave some of their own!