Have kids reflect on the many and complex needs of the body in contrast to their appetites and eating habits. One could go nuts trying to balance eating with the many and changing needs of the body to sustain growth and physical activity. Fortunately, the body has mechanisms that, for the most part, make all the changes and perform the balancing act automatically.
So long as one has a balanced diet (see: balance diet chart images) the body automatically adjusts appetite as necessary to sustain growth and maintain a suitable body weight. Exactly how this automatic system works still awaits elucidation by future investigators.
One thing should be clear, however. Failure to maintain an appropriate body weight will always boil down to a matter of consuming calories in excess or failing to consume calories sufficient to balance with calories (energy) expended.