Whether your family prefers Kraft Mac and Cheese with the powdery cheese sauce mix, or the slightly classier velveeta-style, I wouldn't be surprised to find out if the last serving is scraped out of the pan and every noodle is gobbled up. The best way to stretch a box of macaroni and cheese is to add 1 cup of macroni noodles from the pasta aisle. These are bigger and will possibly have a different cooking time. Here's how I do it:
Mac-and-Cheese noodles cooking time - 8-10 minutes
Macaroni cooking time - 6-8 minutes.
After the water has come to a boil, add in the mac-and-cheese noodles and set a time for 2 minutes. When the timer goes off, add 1 cup of macaroni and re-set the timer for 6 minutes. Test for done-ness when the timer goes off the second time. And don't forget to stir!
Prepare the sauce in the normal manner - it will still cover all of the noodles and you'll have 1-2 more servings! Whether you are feeding more mouths at once or you end up with leftovers, extra food for a low cost is never a bad thing!
This can be done with whatever type of noodles are your family's favorite - I just find that macaroni tends to go on sale more frequently.