I used soft Bernat Blanket Yarn which is very thick with a size 9 / N crochet hook. Of course, it would be easy to knit as well. I actually made it for a younger child in mind, so it really should be a bit longer to tuck into a jacket.
You can use any thickness of yarn with any hook size for this project.
Just cast on enough stitches to encircle a child’s face, join into a circle without twisting the chain and crochet in rounds.