I wondered about that myself-when our kids were 6 years old, and for much longer after that, the rules were the same as they were for me as a kid-eat what's on the table, or don't eat at all. And "fast food" was something to be gotten on a road trip so we didn't have to stop for lunch-not a common food item.
Fast food on a road trip. There are some in this country that pick up road kills. Bambi, Pepe La Pew, Wylie Coyote, Brere Rabbit... Now I know what the fast food joints put in the kitchen.