Tape two pieces of poster board (in your kid’s favorite color!) together to form a tube that fits around your child’s body. (Leave at least 2" of space on either side for comfort.) Cut two 5" holes on either side for arms, about 1" down from the top. Cut two 4" wide strips of black poster board, long enough to fit around the top and bottom of the tube. Draw a wavy line in the center of each and cut lengthwise. Glue to top and bottom edge, leaving about ½” in between the cut sides of each strip. Use letter stickers to spell the color name on front of the body. Match underclothes to crayon color.
Roll posterboard into a 12"-tall cone; trim to level off. Cut three 2" wide strips of craft foam long enough to wrap around base of cone. Stack them together, then glue them to the base. Glue around base, then cover with a 2 ½"-wide strip of poster board to match cone. Tape a piece of thin elastic inside for chin strap.
Cover the crayon tube with felt and cut out armholes. Cut two felt circles (about 7" and 4" around) in a contrasting color and glue to the front of the costume. Cut two more circles from thin cardboard (about 5" and 2 ½" around) and cover with gold duct tape. Glue these to their respective felt circles. Then cut sixteen 2 ½" black felt squares and glue in a checkerboard pattern (two rows of eight) on bottom of tube. Cut three fin-like shapes from poster board, about 12" tall. Fold wide end of each over about ½" and adhere to three sides of the rocket’s body.