Please describe the most prevalent childhood disease?

Asthma. The most prevalent chronic childhood disease is asthma. Prevalence is the total number of children with the disease at the current time. For asthma, the prevalence is about 10% of children. For childhood acute illnesses that go away after a short while, colds are the most common (usually caused by rhinoviruses).
Cavities. The single most prevalent childhood disease is dental caries, i.e. Cavities.
Obesity. Childhood obesity has tripled in the last 30 years and a third of children and adolsecents are now overweight or obese - representing significant negative health consequences.