Ingraining the notion of "dieting" into young children is a dubious notion.  First and foremost, teaching children to "diet" (i.e. monitor what they eat and make sure that it is all low fat or low calorie) creates a complex that they will carry on into adulthood.  Second, it also tends to signify that "dieting" = healthy, and that is not at all the case. 

In a University of Alberta study that appears in the journal, "Obesity," cited in the BBC News, researchers have discovered that giving low-calorie foods to young rats causes them to overeat.  Low calorie snacks disrupt the ability for the animals to instinctively determine how much food to consume during regular meals. The rats end up associating the low calorie alternatives with high calorie foods, and end up eating significantly more calories than they would had they not begun to eat low calorie foods early on.

Similarly, humans who start eating low calorie snacks are unable to determine an appropriate caloric intake when they eat normal meals, and tend to overeat. According to the study, "Based on what we've learned, it is better for children to eat healthy, well-balanced diets with sufficient calories for their daily activities rather than low-calorie snacks or meals."

This is just another reason why ?dieting? too young is a bad thing; the body becomes physiologically incapable of determining when the body has had enough calories.   "Dieting" is a form of training, but their is no better training than leading an active life and eating  a well balanced diet.