Children's Caloric Needs

HHS/USDA Dietary Guidelines for Americans: 2005
The numbers in the chart are calculated based upon median height and weight. 
"Moderate activity"= activity equal to walking 1.5-3 miles per day at 3-4mph
"Active"= activity equal to walking 3+ miles per day at 3-4mph

2,000 is a magic number in the food world, as it is the number of calories upon which food labels base their calculations, but in reality, that number varies greatly based upon age, gender, and activity level, along with other contributing factors, and, unfortunately, 2,000 calories may be more than many should be consuming, especially children*.
Calories are the energy in food.  If we take in more calories than we expend, the calories are stored in our bodies; conversely, if we burn more calories than we consume, we lose weight.  It takes 3,500 calories to equal one pound, so if every day your son eats 100 more calories than he burns, then in a little over a month, he has gained an extra pound, which does not seem bad, until across a year, he stands to gain eleven extra pounds, so an accurate idea of your child’s diet and the number of calories they need can make a sizable difference in their long-term health. 
Want some insight into your child's diet?  As children are creatures of habit, choose the five food and drink items that your child consumes most frequently and study their food labels.  Determine how many servings your child actually consumes and multiply that by the number of calories in a single serving (remember that labels deceive and one bottle of a favorite drink may actually be two servings, so the caloric intake may be double what the label states).  Are those foods alone putting him or her too close to how many calories he or she should consume?  Read the rest of the label.  Are those foods matching calories with healthy contributions to nutrition or are they delivering too much of unhealthy items, reminding yourself as you read that the percentages given beside items like sodium are the percentage of a 2,000 calorie adult diet, and need to be adjusted accordingly.  (Visit the Mayo Clinic's nutrition recommendations for children for sodium and beyond.). 
If you do not like what you discover, take action.  Jot down all that your child eats in a week for a better picture of their diet.  If you are still concerned, make an appointment with your pediatrician to review the food log you recorded and get recommendations for how to improve your child's diet so that it meets the needs of his or her growing body and helps them to thrive.  

Now to get on my soapbox: Though parents need no more servings of guilt heaped onto their plates, we do need to remember that we are in charge of our children's diet.  So if your child "will only eat junk," it is because that is what is being offered, as they do not go to the store and buy their food.  Easily dismissed as being "just kids," children learn their eating habits from their parents, and as they mature and are able to make food choices, not surprisingly, they will choose what they know.  If bad habits are ingrained, then they will make poor choices, potentially setting them up for a lifetime of issues with weight, diabetes, and heart disease, just to name a few.  So before you rush them to the pediatrician because of their bad diet, check out your own plate to see if your dietary choices may need to improve before you can expect the same of your children.

*Nutritional needs are unique to each individual and no radical changes should be made to a child's diet without first consulting a medical professional.