A simple replacement for the drawstring in a child’s garment is a shoelace. It should be the length of the waist or hood of the garment plus sufficient extra to tie. Slip the tip into the opening and push it around to the other side. At each end, make a knot larger than the opening to prevent the shoelace from slipping out.
For an adult garment, you can make a drawstring of cord, fabric tubing, braid, a leather strip, or ribbon, all of which can be purchased by the yard. Attach a safety pin to one end and push it through the casing. Remove the pin; tie a knot at each end.