I agree with JF that teaching a "child" to fast to loose weight is not a good idea because of the long term reprecussion of setting them up for bads habits and anorexia.
Teaching them a healthy lifestyle of good eating habits while not be being the fastest it will be the best in term of the bigger picture and lasting effects. Also she mentions it as I did in my post that starting to juice veggies is a biggie not just for the child but for the whole family.
The theory behind juicing is that the body must turn everything into juice before digesting it. When you juice you've already done 80-90% of the digestion so you body is able to utilize a much greater portion of the available nutrients. So in short drinking a 12 oz glass of veggie juice would prolly equal the same as eating 20 lbs of veggies and that is if you chewed everything real good and your digestive system was in top shape.
I honestly don't know of any other way to supply the body with any better or greater amounts of nutrients in an efficient natural way.
One of the most overlooked things in any diet including those that "go vegetarian" is getting enough "healthy fats" ie: EFA (essential fatty acids). It is widely believed that the lack of EFAs in the SAD(standard American diet) is one of the if not the mayor cause of the obesity epidemic in this country. Check out the links below for lots of info on EFAs