Exercise interventions in children show negligible impact

Unfortunately there's just no way round reducing food intake.

http://www.medicalnewstoday.com/articles/250870.php

Although higher levels of physical activity are associated with lower body mass indices, exercise intervention programs are ineffective at lowering BMI.

The important thing about this review is that the studies did not rely on questionnaires. Questionnaire based studies are extremely prone to error, so this review only included studies using accelerometry devices.

Although previous studies have suggested such interventions don't achieve reductions in BMI because they make children eat more calories, this review found that total physical activity is not increased. In other words, adding extra exercise classes displaces other forms of activity or children naturally compensate by being less active at other times.

"Stop eating so much" is unfortunately the only solution to the problem.