I learned a tip a few years ago on how to get the powdered cheese sauce on your kids' mac & cheese to not come out "lumpy" with chunks of the powdered cheese amongst the macaroni noodles.
After draining the water from the macaroni, put the butter/margarine in the pan with the hot pasta and stir till it is melted. Put the milk and powdered cheese sauce in a small bowl with a lid (one that seals well). Put the lid on it, then shake till all the powder is dissolved. Stir this into your buttered noodles.
Happy eatin', kids! :)