Using dad's hat size as a template, use a measuring tape to get the proper length marked out on your choice of paper, making sure to leave about an inch for overlap. Decide how you want the top to be shaped -- maybe with tall spires or flat, pointy triangles -- and then cut it out.
When it comes to decorating the crown, consider a wide variety of creative embellishments. For example, you can cover it with aluminum foil and glitter to make it shiny and sparkly. You can also cut out shapes from construction paper and glue them on -- maybe hearts, stars or letters spelling No. 1 Dad. Craft jewels can really add some flare, as well as other shiny types of paper or stickers. Young Picassos might also want to paint the crown or color it with crayons or markers.
When all the decorating is done, apply glue to the inch of overlap and press the two pieces together, keeping its crown shape. You can also staple it, if that's easier.