Have a grown-up helper (GH) put fries in oven on a cookie sheet, heated according to package directions.
While fries are baking in the oven, have your GH place a saucepot on the stove and heat it over medium heat. Add butter and melt it. Add the flour and stir. Cook butter and flour together for one minute or so. This is called “roux” (like Tiger and Roo with Pooh Bear) and it helps to thicken up sauces.
Use a whisk to stir as you add milk into the pot. Keep stirring until the milk gets thicker. Use a wooden spoon or heat-safe spatula to stir in the cheese. When all of the cheese melts, take the sauce off the heat and stir in three squirts of ketchup. This is your SECRET ingredient. When everyone asks what you put in your cheese sauce that makes it taste so good, just say milk and cheese – you can NEVER tell what secret ingredient you add! Tell your GH to keep the secret, too, or they can never help you again!
Pour the sauce over the fries or serve the sauce on the side if you like to dip better.