I used this pattern as the basis for a Pikachu hat, using chunky weight yarn and making the smallest size for my 5 year old (he has quite a small head).

The edge of the hat is crocheted around and the face was made by crocheting shapes and adding bit of freehand embroidery. (I just did a Google image search and copied the face as closely as possible). The Pikachu ears were improvised by picking up 12 stitches on the top of the head and and knitting on dpns and decreasing until they looked about right.

Once the hat was finished I decided to line it with fleece to make it warmer - this also has the effect of making the hat keep its shape better and stopping any curling at the edges. I found some yellow fleece fabric and cut out two pieces on the bias in the shape of the flattened hat and then sewed the two pieces together and hand-stitched the lining inside the hat.

The pokeball pom-poms were done by following a tutorial on youtube. It helps to have a pom-pom maker for this, but you can also do it using cardboard.