How long does it take to start losing weight when eating healthy food as a child?

Depends on Variables: The target for weight loss is satisfying hunger with less calories than your body burns off in a day of regular living & exercise. A pound is ~3000 calories of energy, so if you take in 2000/day and burn off 2500/day, after a 30 day period you could drop 5 lb. Salt intake can cofuse the mater with fluid retention. If you eat healthy foods but not less than you burn off you will not lose & may gain.

Depends: It is not food alone need to focus on healthy habit and workout to see weight loss - kids do well if they have growth spurts so use Amino Acids like ornithine, arginine and lysine to stimulate growth hormone.