Once or less. Its ok to eat it as often as you want, but to be healthy, once a week or less is best.
Try not too! If at all possible try to avoid eating fast food, as the high sodium and fat content can change your child's taste buds and makes them crave these types of foods. If you need quick meals try picking up sandwiches at your local deli or getting salads to go. Sometimes it can be helpful to spend one day on the weekend making a bunch of food with leftovers to be used during the rest of the week.