Talk to Pediatrician on Health And Weight

I have 5years twin boys. They were born premature at 27 weeks. But they were very healthy since 3 mnths to 3 years of age. Once they started school, they started loosing weight. Now they are 15.2 kgs and 14.9 kgs. Although they sleep for around 11 hrs a day and eat well almost at every meal time but they look very thin and weak. They are active and cope with their school mates Alhamdulillah but they look weak. Should i give them any multivitamin or iron syrup?? Or is it ok to weigh 15 kgs and look thin??