I don't really get the "no dessert on an empty stomach" thing. Honestly, as someone with hypoglycemia, often I *have* to eat sugar before I can stomach actual food. I don't really see the purpose in making treats only available after meals, if the kid is otherwise eating a well-balanced diet.
Here's what I think actually happens when dessert comes after a meal:
Kid fills up on the meal -- either because she is hungry or because she's told she needs to eat all of XYZ before she can have dessert.
Then, kid sees dessert. She's already full but dessert is sooo yummy... so she has some.
Now she's just learned to ignore her body's hungry/full signal.
If dessert comes before, or during, or separate from the meal, sure, she might end up eating two less bites of broccoli at dinner, but she can better tune in to her body's signals. She's probably not likely to push herself past "full" for just one more stalk of broccoli. She's more likely to stop when she's full.
That's just my theory, I don't know if there is anything to back it up but it's what makes sense to me. We don't keep a lot of sweets in the house, but I don't generally restrict DS's access to what we do have, nor restrict sweets at parties/holidays/etc. There are exceptions. We have a strict "no chocolate in the late afternoon/evening" rule -- more because of the caffeine than the sugar, I want him to sleep! And sometimes I will say no to his requests for sweets, but more because he is supposed to be doing something else right then (i.e. getting dressed etc.) I also keep any treats in the very top cabinets, just to minimize temptation. He's much more likely to grab a banana or apple, they are right out on the counter.
I also like to make healthy "treats" -- things that look & feel like treats but have no added sugar and are made up of things I'd be happy to have him eat even in place of a meal. So sometimes for breakfast he has popsicles made with just plain (no sugar) yogurt and a banana. Snack is occasionally something like a pumpkin pie (made with no crust, minimal sugar, and blended with tofu) or avocado pudding or apple nachos. I think it helps to make it seem like treats are not forbidden, without ACTUALLY having him eat sugar constantly.
Anyway, seems to work for us. He is 3 1/2 and eats just about everything under the sun. And he will stop licking a lollipop (special Halloween treat!) to have some bites of veggies -- completely of his own accord!