I'm a teacher, and I absolutely refuse to police lunch boxes. There was no part of my teaching degree that said that I was required to monitor what every child in a class of 30 eats each day.
My own DS started school last week and each day he has had a some variety of a sandwich, rice cakes with vegemite, pasta salad, yoghurt, apple, banana, strawberries, grapes, sultanas, homemade zucchini slice, homemade cornbread. But he has also had a small container with a small treat - either half a dozen Freckles, a jelly snake, a homemade mini cupcake, a homemade chocolate crackle. He eats everything I give him, and if he were to not be able to eat his treat if he wanted it, then I'd be furious.
A child with a healthy diet should not be told that certain foods are 'bad'. But a balanced diet is what should be discussed with children.
I'm an adult, and I take a mini chocolate bar, or a piece of cake or brownie, or whatever I have cooked, with me for my work lunch. 'Dessert' is not exactly a difficult concept, and some people enjoy a sweet item to finish their meal. A small sweet item consumed after a healthy meal is fine, and should not be treated like the child is doing something wrong.