The most common reason to be in situation like this is because he or she knows that he or she will get sometimes to eat latter. While a child should not be forced to eat, he should learn his main meal in the evening and will be served at dinnertime.
Make sure that they don't reach out for a snack after school or drink too much juice, soda or even milk before dinner. Don't offer any rewards or bribe for eating.

These are just approx values to give parent an idea of what the caloric requirement for children are. Individual variations can exist and parents need to determine what and how much is best.
Children below 1 -3 years of age need 1100 to 1300 kcals, 4 -6 years need 1600 to 1800 kcals and 7-6 years needs app 2000 kcals.
After this age, boys and girls have different caloric requirements
Girls between 11 and 18 years of age needs anywhere between 1800 to 2000 kcals and 11 - 14 years old boys needs about 2500 kcals.

As the child grew up, they need good amount of variety of food to meet his/her nutrients demand. As long as they are gaining weight and have normal activity level, then you have little to worry about. You can still offer variety of food.

Grain soup serving include 1 slice of bread, 1/2 cup of cooked rice or pasta, 1/2 cup of cooked cereal. Child should 6 servings from this group.

Fruit group include 1 piece of fruit, 3/4 cup of 100% fruit juice, 1/2 cup of canned juice or 1/4 cup of dried fruit.

Milk group include 1 cup of milk or yogurt or 2 ounces of cheese.