I often receive requests for my hat patterns to be redesigned for another size head. I am happy to do this from time to time, but I would rather you dear reader gain the skill for yourself, so you may have total control over your knitting. So let's get started.
Say the hat pattern you want to follow is this one. The pattern is written to an 18 inch head, but you want to make it for your two year old. Measure the circumference of their head with a tape measure. You hold the tape measure all the way around their head and over their ears. Don't pull tight unless you want a very snug fit. Let's say you get a measurement of 16.5. Okay, now what? Multuply that number by the guage for the stitch pattern. Let's say it is stockinette and you are getting 4 stitches to the inch. Multiply 16.5 times 4 which equals 66 stitches. You cast on 66 stitches rather than 72 in the pattern.
Now let's make that crown fit.
Take the tape measure and measure the distance from one ear to the other ear up and over the top of the head, sort of like a headband. Let's say the measurement you get is 15, divide that by 4 which equals 3.75 inches. So you follow the stripe pattern as set in the directions which was basically one inch of a color before switching. When you get to 3.75 inches from the brim edge, you begin your decreases. This should give you a nice form fitting beanie.
For a firm band, knit on a needle size 2 sizes smaller than the pattern calls for, when the brim is complete, switch to the larger needles.
head circumference x number of stitches to the inch from gauge swatch = number of stitches to cast on
crown is measurement from ear to ear divided by 4, knit hat to this number before beginning decreases.
I hope this helps. Please let me know if any part of my instructions are not clear.