Hello & welcome to Just Answer! If you have any questions after reading my answer, be sure to let me know.
It's good that we are paying attention to the weight & especially the belly fat in this age, since you are probably aware that belly fat can contribute to a lot of health problems including Diabetes, hypertension, heart disease etc. over the next few years.
The best ways to reduce belly fat are actually the same as for general weight loss
1. Dietary Changes - Eat such nutritious foods as fruits, vegetables, whole grains and low-fat dairy products; since they tend to be relatively low in calories, such foods can help reduce your caloric intake.
2. Fast Food - Limit your fast food intake -- this can go a long way toward trimming your waistline.
This also includes reducing cold drinks intake since these add a lot of empty calories
3. Eat a good healthy Breakfast ... Do NOT skip it
4. Limit Screen Time: TV, mobiles, computers can all eat into the productive time that can be used to promote a healthy lifestyle ... reduce it to less than an hour on weekdays, and a couple of hours on the weekends
5. Increased physical activity - walking is great BTW! Just try to make it as brisk as possible .... Also competitive sports may provide an additional impetus for some children. Dancing works as well for kids not too sporty by nature (my sister is an example of that :)
We need to build up the stamina and this will take time
To tone up the abdominal musculature, concentrating on abdominal exercises may help in getting the tummy trimmer faster in addition to the general weight loss.
What I would recommend is interacting with a physical trainer to get the right technique for exercising the belly muscles,
You can get a few pointers here
Please get back for any clarifications,
Dr Gupta, MD