Maybe they are allergic to that kind of food, eg. peanuts, shell fish or cow's milk.Maybe their bodies can't digest that kind of food, eg. gluten, which is found in wheat.Maybe they have a condition that needs them to have a special diet, such as the ones below.
Kids with this condition need to limit or not eat at all, any product that has milk or dairy products in it. Lactose is the sort of sugar found in milk products.
Diabetes (say di-a-beet-eez)
Kids with diabetes may need to pay extra attention to the amount and types of carbohydrates they eat, because they have to keep control of their blood sugar levels to stop them getting sick.
Coeliac disease (say see-lee-ak)
These people need to have a gluten-free diet. Gluten is in many foods and it was difficult to find what to eat until recently. Look for the gluten-free products in the supermarket and look out for gluten-free recipes that you can make at home.