The type of food your child eats can make a difference. As time goes by I'm sure we will learn more about how the body handles different foods, but even though there are subtle differences in how the body metabolizes various foods, for the most part weight control boils down to the balance between calories consumed and calories burned. There is no such thing as diet food - food that your body somehow uses differently to magically lose weight. Eating traditional diet foods cannot guarantee success in weight control. She was right, but there are important differences between foods.
Some foods are clearly better for our health.
Cardiovascular health benefits from a diet low in saturated fat and cholesterol. Low fat diets with lots of fruits and vegetables provide some protective effect against some cancers. Weight control can benefit from foods that fill you up with fewer calories. Foods high in fiber and water content can also help to fill a stomach with less calories. These foods include fruits and vegetables.