For the walnut shell, cut an oval shape from cardboard that's big enough to fit over your child's back for the walnut base. Use brown kraft paper and crinkle it up to create the volume of half of a walnut shell and hot glue it to the cardboard base. Cut a large sheet of kraft paper crinkle it in vertical wiggly lines like a walnut shell. Use brown paint or markers to draw in the lines if you like too. Glue the edges around the whole walnut shell base. Repeat to make another walnut shell for the front of the body. Cut two tan felt straps to go over the shoulders, and glue them to the top edges of the shells.
Alternately, if you want to just have one half of a walnut shell on the back, you could make the costume as an open walnut in the shell. Cut a walnut shape from tan felt and use brown paint to paint the shadows of the wrinkles and curves of a walnut. Attach it to the front of the pj top with double sided fabric tape.