String, scissors, tape, construction paper, pens, crayons, and anything you would want to decorate a crown with, such as perhaps glitter and glue.
Using a piece of string, measure your child's head and cut the string to fit the size. Then use the string to measure a piece of construction paper; you might need to tape 2 pieces of paper together if your child's head is on the larger size. Once you have the correct length of the paper, cut one side (lengthwise) into a design that resembles a crown. Then, have your child decorate as he or she wishes, either just with markers or crayons or 'fancier' with glue and glitter, sequins, beads, craft jewels, etc. After the crown is decorated to your child's satisfaction, place the crown around his or her head, and once at the correct size, tape it closed. Make one for yourself too!