Basically what you will need is denim overalls, t-shirts and hats in red and green for Mario and Luigi respectively. Cut circles from yellow felt and draw an X on each one with a black marker. Glue or sew these on the overalls, to represent the buttons. Use white paper or felt to make the details for the hats. You can even print these to make it easier. A big M for Mario and L for Luigi. Attach these to the respective hats using glue.
For a final touch, draw mustaches on your little plumbers and they are ready to go.
This Halloween costume idea is perfect if you have two little boys and want their costumes to match.
Courtesy of https://www.instructables.com