Does anyone have any ideas or tips on making Bowser and Yoshi costumes? Those are what my nephews want to be for Halloween.

If you get those paper cones and cut a beachball in half you can make Bowser's shell... Assuming they are still small enough to have a beachball be the right size.

You could probably also sculpt a basic form from wire and cover it with cheap fabric that you then paint. Just remember that the face needs to be clear enough for peripheral vision, so instead of a full-face mask just make the snouts, and be sure they have sufficient air holes for breathing and speech.