Before putting your kids on a diet, please remember that you should not put your child on a fad diet. They are likely to end up deficient in minerals or vitamins which could cause health problems later on, especially those diets that restrict food groups.
You don't need to spend money to put your child on a diet - all you need is to cut down their food intake (that is, the amount of calories) and increase their activity levels. Generally, if you swap out the high fat foods in their diets and replace them with fresh vegetables, grains, or pulses you will already cut out a large portion of calories from their diet so you are half way there, so typically you can devise your own free healthy weight loss diet for your kids.
If they are not particularly active at the moment, check how long they spend on a computer or games console per day. This is one of the reasons for so many children being obese at the moment. You shouldnt stop them from playing video games altogether, but you should try and swap at least 30 minutes to an hour of activity a day if you want the most suitable healthy diet for your kids. You could enlist them in school clubs if anything takes their interest, or do some exercise with them - swimming is a great all round exercise and walking/cycling is great for the whole family too.
If you child is stubborn about giving up their games time, then you might want to look at letting them play games once they have met their quota - consider it a reward for doing well. This may need to be done over time as if they have been particularly inactive for a while they are likely to have their lifestyle as a habit now and may be harder to persuade them to change.
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