No fried food
Have a look at the menu and try to refrain from ordering anything that is fried in oil. Most fast foods take pride in their dishes that are soaked in oil. Whatever you order, make sure that such items are avoided in any case - try looking for things that are baked, or steamed.
No supersized meals
Supersized meals are the worst thing you can do to your kid. Fast food is unhealthy enough, and supersized meals are just bigger versions of bad foods. They also have more French fries, and a larger amount of soda - never order these for your kids.
Healthier fast food options
There are fast foods that offer healthier menus in general, or at least have some menus that are not all about fats and carbs. Select such fast food restaurants for your kids, and stress that they eat healthy even when they are at a fast food.
Remove those fats
There might be extra fats in the burger that you have chosen for your child. Some of these foods have rich dressings that have a ton of fat in them, while others contain extra fat-laden meats, like bacon. It is best that you request the waiter to keep all the extra fatty elements out of the burger - if the burger has already been served, remove these elements yourself. This will save you and your kids from a lot of unhealthy calories.
Enjoy the food
Enjoy every bite and never eat in a hurry. Research has shown that people who chew more on average eat less. It also allows you to taste more, and helps in the process of digestion as well. If kids eat in a hurry, it is highly likely that they will eat more.
It is best if kids drink water instead of soda with their fast food meal. This will significantly reduce the amount of calories they will consume, and will be better for their health in general.