I can think of lots of reasons why not. It's not so much the content of the meal but the context of stuffing a child with food to keep it quiet.
If you are weight watching a child eating some high calorie burger and chips is not going to help other slimmers to keep their resolve.
What is wrong with feeding the child a ham butty and some banana.
"Calories in Mcdonald's Happy Meal Cheeseburger, Fries, Diet Coke
Nutrition Facts Mcdonald's - Happy Meal - Cheeseburger, Fries, Diet Coke
Servings:
Calories 528 Sodium 790 mg
Total Fat 23 g Potassium 0 mg
Saturated 7 g Total Carbs 61 g
Polyunsaturated 0 g Dietary Fiber 5 g
Monounsaturated 0 g Sugars 7 g
Trans 1 g Protein 18 g
Cholesterol 40 mg
Vitamin A 6% Calcium 20%
Vitamin C 3% Iron 15%
*Percent Daily Values are based on a 2000 calorie diet. Your daily values may be higher or lower depending on your calorie needs.
and .......
Another source says.
"Depending on their age, size, and activity level, toddlers need about 1,000-1,400 calories a day. So in one meal as described above a child would have half it's daily calories in one meal."
Now once in a while that's not going to do any harm, but it appears a lot more people nowadays are eating more calories than they need and many of those appear to be from fast food in a generation who have had greater access to such food than we ever did.
Which sits with the thoughts above about less people having a clue or the enthusiasm about how to prepare a meal.