Bring a large pot of water to a boil for cooking macaroni. Mix cornstarch, mustard powder and ground red pepper in a small bowl.
Cook macaroni in the boiling water for 5 minutes. Add brocoli to the water and cook until the macaroni is almost tender, 2 minutes longer. Add carrots and stir to submerge. Immediately, drain the macaroni and vegetables. (The macaroni should be tender, but still quite firm; it will continue cooking while you make the sauce).
Return the macaroni and vegetables to the pot. Add the cornstarch mixture and stir to coat. Stir in milk. Place the pot over medium-high heat and cook, stirring, until the milk comes to a simmer and thickens, 2 to 3 minmutes. Remove from heat and add Cheddar cheese and Parmesean cheese; stir until melted and smooth. Season with salt and pepper.