At our house, the parents serve the children's plates. We put a very small portion (as in 1 level tablespoon) of each food on each plate and give them each a cup with about 4 ounces of milk for each meal. (Do you think I put "each" enough times in that sentence?)
The children know that when they have eaten everything on their plate they may have seconds of anything they wish.
This saves us from throwing out good food or pouring milk down the drain. It also helps when there is a spill, and you know there will be. It is not so bad to clean up a tablespoon of spaghetti off the floor or 4 ounces of milk. An entire plate of spaghetti or a full 12 ounces of milk is altogether another matter.
I'm often shocked when I see parents filling up their children's plates to the brim with food and giving them a full glass of something to drink. Inevitably, at least half the food gets thrown in the trash and maybe 2 or 3 sips are taken from the glass.
I wonder if parents are used to the portions served at restaurants and so they fill up their children's plates in a similar way and then urge them to "eat, eat, EAT!" We don't eat out often at all (as in 2 or 3 times a year) and I hate to throw good food down the drain. For that matter, I can't afford to throw good food down the drain.
Keeping the portions small and then allowing for seconds is a good way to keep food waste to a minimum.
Check out Biblical Womanhood for lots of great frugal tips.