I wrote up a set of instructions for a square hat, made in yellow acrylic yarn.
Included with the pattern are additional instructions to embroider and add yarn and felt pieces for the Instructable Robot, Sponge Bob Square Pants-type-of-look hat, and WALL-E type-of-look hat.

The hat fit my large-headed 3-year old son, my 4-year old & 8-year old and adult self. It was tight around my large-headed husband, but it fit him too.

I used acrylic yarn for the hat and the crocheted details. I used Lion Brand Bonbons to embroider the felt details. One skein of super saver yarn should be sufficient for two hats.

You will also need a yarn needle, embroidery needle and various colors of felt if you are going to make the Sponge Bob or Wall-E type hats.

Please email me on the Instructables site if you have any questions and thanks for reading - DeAndra with DeAndrasCrafts.