If you choose carefully, fast food restaurants do offer nutritious choices. Dietitians tell me a good choice is a cheeseburger. It provides protein, iron and calcium for your child. Stay away from the bigger burgers though. They can have twice as much fat as single cheeseburgers.
French fries can also fit into a healthful diet, but try to avoid upsizing.
Larger orders of fries can sometimes have three times more fat than small orders. Your children could even share an order of fries. Another good choice is a grilled chicken sandwich. You'll add more vitamins if you order with lettuce and tomato, but skip the mayonnaise and special sauces. Many children would rather have a chicken piece, that's fine.