The shortest answer is “you don’t”. Not in the sense we think of it, anyway. Diet is a word that has changed over the years and now is almost always filled with negative connotations. The word diet should mean the kind of foods we eat. I have a vegetarian diet, others have a meat based diet, and other still have rice based or fish based diets. Much has to do with climate, geography and culture (or religion).
We tend to look upon the word diet as in “I have to go on a diet” which can really mean anything. But we generally take it to mean we need to reduce our calories and embark on a different way of eating because we're fat. We don’t like it, the thought of being on a diet creates dread (as well it should, since most “diets” are fad diets that simply don’t work) and they can create false expectations and eventual depression that nothing will ever work, and it’s back to eating badly as usual.
Our children are in great danger of becoming overweight or obese. Looking around the classroom, we now see plenty of overweight kids, when even 20 or 30 years ago most of us were pretty slim and fit. These days things are so different. We are choosing processed foods for our children because it’s faster to make or may only need to be heated. Despite the fact that healthy eating is not necessarily more expensive, we have become used to eating processed foods and often on the run (often to endless extracurricular activities) and when children are home, they are playing video games, watching TV, on the computer – doing anything but moving.
We can blame a million things as to why so many of our children are in the state they’re in, but much of it comes from parents who are as unmotivated as their kids. We too are on our video games and watching TV (all while claiming to be busy) or playing Farmville and other “addictive” social media games. We all have only 24 hours in a day. Some of us definitely have a harder time keeping things together; multiple jobs, no access to decent supermarkets (especially in urban areas, ironically) and little money – but it can be done.