Go to your local thrift store to search for medical scrubs, a lab coat and a stethoscope at discount prices. Purchase a plastic stethoscope at a toy store if you cannot find one at the thrift store. Buy a lab coat online if you can't find one in stores, and substitute dressy slacks and a button-down shirt for the scrubs if those cannot be located.
Iron each of your costume elements with starch, so they look professional. Make a red cross on the front pocket of the lab coat with two pieces of red electrical tape. Draw the cross directly onto the coat with a red marker if you do not have tape.
Put the stethoscope around your child's neck. Fill the canvas or computer bag with the contents of a first aid kit, such as gauze, bandages and surgical tape. Put the small pad of paper and a few pens into the lab coat pocket, to use for writing prescriptions.