Here is a list of ways sugar can affect a child's health or anyone's health, for that matter::

Sugar can suppress the immune system.
Sugar can contribute to hyperactivity, anxiety, depression, concentration difficulties, and crankiness in children.
Sugar can cause drowsiness and decreased activity in children.
Sugar can cause hypoglycemia.
Sugar can cause kidney damage.
Sugar can increase the risk of coronary heart disease.
Sugar can raise adrenaline levels in children.
Sugar can lead to periodontal disease.
Sugar can contribute to weight gain and obesity.
High intake of sugar increases the risk of Crohn's disease and ulcerative colitis.
Sugar can contribute to diabetes, osteoporosis.
Sugar can cause cardiovascular disease.
Sugar causes food allergies.
Sugar can contribute to eczema in children.
Sugar can increase the amount of fat in the liver.
Sugar can cause depression.
Sugar can increase the body's fluid retention.
Sugar can cause hormonal imbalance.
Sugar can cause hypertension.
Sugar can cause headaches, including migraines.
Sugar can cause an increase in delta, alpha and theta brain waves, which can alter the mind's ability to think clearly.
Sugar can increase blood platelet adhesiveness which increases risk of blood clots and strokes.

See below site for more ways that sugar can effect health.

it might cause someone to get bullied