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

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.
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.