Draw your desired "key" shape on the cardboard. Use a utility or manual knife to very carefully cut out the shape of the key from the cardboard.
Trace the key shape onto another piece of cardboard and cut it out again, giving you two cardboard replica shapes.
Spray paint both cardboard keys in a well-ventilated area. Follow the instructions on the side of the spray paint can for specific information. Let the keys dry..
Cut two pieces of elastic to 15". These will be used as arm straps. Glue both ends of one of the elastic pieces to the inside end of one of the braces, creating a loop. Repeat for another piece of elastic and another cardboard Allow Let it dry so the elastic stays firmly in place.
Drip glue all over the inside of a key, making sure the glue is on the same side as the elastic. Place the other key on top, sandwiching the glue and elastic ends in the center of the two keys.
Dress your child in an old-fashioned outfit and then slip the elastic of the keys around his shoulders like a backpack..