You have to estimate how many of those meals you hold. You probably have to buy more hot food than you will serve. The canteens will waste a fair amount of hot food, and quite a few sandwiches.

So, if the sandwich is not cost effective, and it isn't even practical without wasting food, then the only reason left is to punish, and it is punishing kids, not parents.

What if the reason the parent doesn't pay is that they are already neglecting their kid? What if that hot meal was the only hot cooked meal their kid got?