Do people not REMEMBER being a child?
Children are not 38 year old administrators. If they don't want to eat an apple because they'd rather go and play, they won't eat the apple. They have no concept of scurvy, or of learning by what has happened to other people they don't personally know.
My brother started eating fruit after a bout of constipation nearly turned him inside out - he was 5. I started eating things other than bread and ketchup when I went to school and was nowhere near my mother - who was under no illusions that I ever ate the fruit, because I just used to leave it in the lunch box. For Days.
38 year old administrators eat fruit, and say things like "Really, green vegetables are so much nicer when they are crisp, don't you think, Jeremy?" and they go to the gym and sweat occasionally and they never eat foam prawns until they vomit pink froth.
6 year olds are not the same breed. Many prefer to live on sausages, garish shaped potato products, false coloured dips and side dishes, and sweets. It's not a matter of bad environment, you could put even the greediest of 38 year old administrators in the position of only having sausages, garish potato, weird side dishes and sweets, and because their tastes are innate, they will tire of the diet rapidly - well this goes for small children and virtue food.
Children like salt, they like sugar, they like packaging and garish orange food shaped like unlikely objects, and plain plain plain carbohydrates. It is our job as adults to steer them towards a more species appropriate diet, but we mustn't expect them to take to it immediately. It's nice when they do, it must be satisfying to see your preschool children chow down on sage marinated mussels on a bed of bitter romaine and lollo rosso, but it's unlikely to happen immediately, with the best will in the world!