In reality, if your child is overweight or obese, you do have a cause for worry. The earlier in the childâs life that you make changes a childâs lifestyle in regards to nutrition and physical activity, the easier it is. Measuring Obesity in Children. You may be confused with the many new terms related to obesity. There is good reason for this confusion as there is controversy over the definitions related to obesity. For adults more than 18 years of age, the definitions are based on BMI.