Draw or find an image to serve as a pattern for the costume. Look at a package of Kool-Aid for inspiration.
Measure the child being fitted for the costume. Create a wire mesh of an appropriate matching size to give the costume girth and substance. Shape it into something recognizable as a large pitcher of liquid (the Kool-Aid Man).
Attach a few inches of foam padding to the outer portion of the wire using fabric glue. Concentrate the padding around the lower portion and the stomach area of the wire mesh to provide bulk. Leave the upper areas free for arms and be sure to leave two large openings on either side of the wire.
Cut a large piece of red fabric to wrap around the outer portion of the mesh/padding. Size will vary with each costume, but in general, you'll need about a 5-foot-by-5-foot piece of fabric. Check the size of the fabric by wrapping it all the way around the wire mesh so that it covers all of the outer portion.
Wrap the fabric around on top of itself in the size and shape of the wire support and sew a seam down the entirety of the ends, making the entire thing one piece. Slip the fabric over the wire/padding and secure it by sewing the fabric and padding to the mesh by hand in several places. Cut two holes for arms on either side, matching the same areas in the mesh.
Paint the child's face red. Highlight the area around the eyes and mouth with black face paint in the manner of the Kool-Aid Man.
Fit the child into the costume. Look for any areas that need slight alternations or adjustments. Avoid bunching of the fabric by sewing down any bulging areas to the supporting mesh frame.