Oh yes, jura, I'm not saying there aren't unhealthy food choices, especially such sugary things as you mention. But there is a lot of nonsense talked about perfectly healthy food as well, especially, in my opinion, with regard to animal fats. As a parent I have the right to feed my child butter rather than low fat vegetable marg for instance, if I so choose, and it's no-one else's business.
I'm in two minds about the children who are given too much sugar and not enough in the way of vitamins and minerals in their diets. I think a school's role should be limited to educating children about food values and a balanced diet, without being too restrictive. The aim of that education would be to give them the means to make good choices (if they so choose) when they are in charge of their own food.
I'm really not sure about interfering in what parents do, even when the parents appear to whoever is judging to be incompetent. The easy answer is educate the parents as well, but of course there never is an easy answer. To my mind there is no need for anyone who can read to be ignorant of the basic food values of foods – calorie content, protein, carb, fat, minerals, vitamins, etc. The information is everywhere!
I do it too, but I really think we should try not to judge too harshly people who do things differently from us.