Firstly, I wouldn't suggest trying to lose weight "fast" is a particularly goal. Instead the child should look into a structured way of changing his or her lifestyle..
Key to healthy living is what you eat and exercise. You can start with an intense detoxification-style cleanse which may result in a dramatic loss of weight and push you into the initial healthy living mindset.
But there is a danger that one will get frustrated and give up.
It is better to eat healthily but with a few concessions than to subsist on nothing for a week and then
splurge on junk food after getting upset by the restrictions.
For a child it is important to speak to a youth dietician or school nurse to get a professional opinion
since individuals differ in terms of what they need.
Eating lots of fresh fruit and vegetables is the best place to start; try to stay away from processed food and ready meals which often have high fat, salt and sugar levels, and stick to cooking fresh ingredients, which will contain more nutrients.
Exercise will depend on the individual because you need to find suitable activities in order to carry on
with the regime.
It is a good idea to get him or her involved with team sports because they can provide
the encouragement needed to keep up with the activity but, again, this will depend on the individual.
Maybe they would prefer something non-competitive such as yoga, which will provide real wide-ranging health benefits.