Nothing beats a clean, pure and wholesome diet. Even various food pyramids will all generally agree about a basis in vegetables, fruit and complex carbohydrates. Proteins should follow.
It seems to get more difficult from there.
Ask any parent about picky children’s diets, and there is easily a phase where it seems like kids can eat well, but also be overloaded in simple sugars, starches, and snack foods that may lack nutrient density.