Just like with planning, if you go to a restaurant / fast food restaurant with your child you can do incredible damage control if you do a few things in advance.
If you can, check out the website of the restaurant. You would be surprised to see the differences in calories between two hamburgers can reach hundreds! Would you rather pick a meal for your child or yourself which has 500 calories less? Would you consider sharing an order of French fries?
Check the children's menu, it is mind-boggling that these items are all fried, include sugar, no vegetables and fruits. Agree with your children in advance that you will substitute some items and be smarter than the restaurant.