Childhood obesity is, by definition, a condition in which a child is significantly overweight for their age and height. Childhood obesity can lead to lifelong health issues, including diabetes, high blood pressure, and high cholesterol levels. Research shows that parents should address their at-risk child's eating and activity patterns early rather than delaying in hopes that the obesity and the habits that support it will resolve themselves as the child gets older.
This article from the Mayo Clinic provides an excellent overview of the symptoms, risk factors, complications, and prevention of childhood obesity. Of course, concerned parents and caregivers should always contact their child's pediatrician for a proper diagnosis and plan.