From our friends at HelpMeGrowUtah.org:
As your children grow, their bodies and nutritional needs change! The rates their bodies are growing change, and this correlates to their hunger and nutritional needs. After about two years old, a slowing down in the rate of physical growth comes with a reduced appetite. Each child is different and needs different amounts of nutrients to keep them healthy. How much they need to eat also depends on their level of physical activity and their metabolic rate.
Visit their blog for steps to help children be at a health weight and learn health habits.