I was told, as a kid, that it was rude to not eat all the food put in front of you, especially at other people's houses. Other parents would comment on how polite we children were for clearing our plates.
This does not make much sense to me. If eating all your food is the only alternative to throwing food out, it is sort of OK, but even then, it does not solve the issue at all because you are the one eating all the food, not the starving kids in China.
Additionally, why come up with such nonsense? This is one of the worst things you could do to your kid. You should instead educate them and tell them the actual reasons for why one should not waste food.
Mind you, if you cook less instead, then you would not have to eat them, nor throw them out. :)
I am not sure that this is how I would teach my kid to be grateful, especially because the kid might perceive it to be forced and might backfire in the future.
Anyways, we might be overthinking this. :)
I assume you are only responding to "the kid would not need any reasons to be told if s/he is hungry.". It is true in most cases, I was not accounting for depression and so on, but that in itself might dissipate hunger.
Most toddlers will eat when they are hungry, they will not just go hungry. Power play or power struggles complicates things, I deliberately did not bring it into this. Regardless, I still do not see that it is necessary to come up with reasons that are completely wrong or silly. :)
Still completely unhelpful to the starving, and still orthogonal...
Eating everything on your plate indicated that you were ravenously hungry, taking advantage of your host's hospitality.
Leaving just a little uneaten meant that you ate as much as you could because you found the food delicious.
Me: Got a stamp?
Mom: <SMACK!>