How can I help prevent my overweight child from getting diabetes mellitus type 2?

Weight loss. The best was you can prevent your child from developing diabetes is to help them lose weight and return to a normal weight for their age, sex, and height. Controlling their diet and encouraging regular physical activity will help with this.
See below. The best method, and most important is weight loss. Which needs to accomplished through a proper diet and regular aerobic exercise. Other than those, there are no other preventative options.