My both son have stomach pain daily, but for Few...

is it because of food or desease ?
those symptoms suggest it only cause by unhealthy or unsuitable diet and food , no signs of disease or infection 
current advices :
1. drink plenty of water 
2. avoid oily food 
3. avoid heavy meals 
4. avoid processed meals 
what symptoms suggest infection or disease ?
1. prolonged pain 
2. fever 
3. dirrhea or constipation
4. dizziness 
5. nausea and vomiting