If you measure dietary intake then you will be able to corelate the intake with the nutritional status of the child. 

Abdulwali Mareh, 

This sounds very topical, especially in Arab countries where little is currently known. I would certainly suggest measuring dietary intake. However, this is not the sole factor influencing weight status. Dietary intake is merely one piece of a huge jigsaw. You could extend your research to look at: 

-Dietary habits in and out of the school environment

-Parental habits - or influence of household members

-Food availability (again in and out of the school environment) 

-Climate and dietary intakes

Please note these are only a few avenues. If you visit the foresight report 2007 (an English report) this details the many pathways to explain weight status. Within this you may find an area you wish to explore further. 

Dear Abdulwalli

There are a myriad of factors affecting the body weight among children. but i have a good suggestion. you can measure their body composition which may be a strong indicator of their calorie intake (percent body fat. muscle mass...). let me know if need more exact onformation. 

In addition to dietary intake, you will need to evaluate physical activity, since this is one of the important factor that can affect body weight.

Beck physical active questionnaor, apprtite questionnair are also applibanle in your thesis. 

As body weight (anthropometric assessment) is a key to assess nutritional status. and another major criteria is dietary assessment. so you take both for your study, then your thesis will be more impact full.