So many things. Obese children are likely to be obese adults. They are far more likely to experience a wide range of physical problems including diabetes, heart disease, stroke, etc. In addition, they are more likely to experience social stigma, be bullied by peers, have lower self-esteem and be depressed. There consequences of childhood obesity are profound.
Long term risk. Childhood obesity contributes to long term risk when it persists through to adulthood. Without intervention, about 80% of obese children will remain obese as adults. Long term studies show 4-5 times the risk of diabetes and about 3 times the risk of heart disease for obese children who remain compared to lean adults. Severe obesity causes GI, breathing, bone, and other problems in childhood.