Corn starch is your friend.
Forget all that Bechemel and roux business (Bechemel is not actually a cheese sauce -it's a white sauce, adding cheese makes it Mornay or something like that)
Heat milk in a pan until just starting to boil. While it's heating, dissolve some corn starch into some cold milk in a cup. Remove the pan from the heat, gently pour in the cold milk/corn starch mix stiriing all the time. Return to heat, keep stirring and bring back to boil. If you don't stir enough it can get a bit lumpy, but that's easily countered with the use of a balloon whisk. It will not be grainy. When it's thickened and is making volcano-like bubbles, turn off the heat and add grated cheese af any type that takes your fancy. Sharp cheddar is good for a really cheesy taste. You can season at any point. And if the kids want it Kraft day-glo orange, add food color at this point
Mix with cooked mac.
To get really fancy, sprinkle more grated cheese and home-made breadcrubs on top, and a little black pepper and shove it under the broiler until it starts to bubble and brown.
To get really really fancy, mix in precooked bacon, peas, onions lightly fried in butter......anything.