Is the old ‘starving kids in Africa’ bit, a good way to get the kids to eat their dinner?

The way my mother gave that line (it was China back then) , she made the point that it was a luxury to be able to say "I don't want food" and I ought to appreciate our position in the world, compared to others who have so little. I would say that her approach was effective -- maybe not so much in getting me to eat my spinach, but in communicating that larger appreciation.

As for being too full for the entree but hungry for dessert, there is actually a biological rationale for that: hunger and appetite are associated with two different areas of the brain.