Grain-based desserts and pizza as well as soda, sports drinks and energy drinks make up the top sources of calories for kids age 2 to 18, according to the publication "Dietary Guidelines for Americans, 2010." With those stats, it's not so surprising that the obesity rate in children has risen from between 4 percent and 6 percent, depending on age range, in the early 1970s to between 10 percent and 20 percent by 2008. A diet laden with junk food not only affects weight but other facets of a child's life, too.

Effect on Mental Health

The need for a good diet begins early. A study published in 2013 in the “Journal of the American Academy of Child and Adolescent Psychiatry” examined the diet of children age 6 months to 5 years old, as well as the diet of the mother while she was pregnant. Researchers concluded that the diet of both the pregnant mother and the child after birth can have an effect on mental health of the child; a diet high in unhealthy junk food and low in nutrient-dense food were linked to behavioral and emotional problems, including anxiety and depression.

Effect on Obesity and Disease Risk

A diet high in fast food in those age 4 to 19 increases the risk for obesity, according to researchers who published a study in 2004 in “Pediatrics.” They found that children who ate fast food consumed more calories, more fat, more carbohydrates, more added sugars and more sugar-sweetened beverages than those who did not. Additionally, these children consumed less milk, less fiber and fewer fruits and nonstarchy vegetables. According to the Centers for Disease Control and Prevention, obese children are more likely to have high cholesterol or high blood pressure, both risk factors for cardiovascular disease. Additionally, obese kids are at higher risk of prediabetes, bone and joint problems, sleep apnea and social and psychological problems. Their obese status can follow them into adulthood, too.

Effect on Academic Performance

A diet high in junk food can hinder your child’s success in school. In 2003, researchers compared the diets of more than 5,000 students to their scores on a standard literacy assessment. They found that students who consumed a low-quality diet did worse on the standardized test. Results were published in 2008 in the “Journal of School Health.”

Ideal Diet for Children

The ideal caloric intake for a child varies based on gender, age and physical activity level. Generally, a girl needs between 1,200 and 2,200 from age 4 to 13. When she reaches her teen years, she needs 1,800 to 2,400 calories. Young boys need 1,200 to 2,600 calories during preadolescent years and 2,000 to 3,200 calories a day as teenagers. These calories should be made up of foods low in saturated fat, trans fat, cholesterol, salt and added sugars. Choose a wide variety of foods from all the food groups, including lean protein such as poultry, fish and beans, low-fat dairy products, whole grains and plenty of fruits and vegetables. Double-check that serving sizes are appropriate for your child's age to avoid overeating.