Bottled water is good also make sure milk is pasteurized. Be careful with fresh fruit, peel or better cook if possible. Take a supply of meds, children's Tylenol, pedialite or powdered Gatorade if they do get sick it helps to keep electrolytes in balance. I also sent the second time motion sickness medication I don't the dosage for four year olds. Try to avoid the times of heavy street traffic as the air makes is hard on their little lungs.
Milk is good to help settle the stomach.
I'm glad my son was able to visit he has made three trips to Iran and will be 16 this summer and is looking forward to going for his four time. I have a daughter who is 10. I have taken her to Iran when she was 3, and also when she was 6. She never got any virus or sick. She was fine.