Step #1: I started with the turtle neck and an old ribbed knit tank top. Cut off the neck of the turtleneck just about the seam. Then line up the top with the bottom hem of the tank- pull the turtleneck so it's taut (as wide as it can stretch). Mark the tank top for height and width, and cut. You'll end up with one long rectangle from your tank.
Step #2: Place the right sides of the knit tank together. Sew the raw edges to create a circle. Now put the circle knit fabric over the sweater neck. The right sides are together with the wrong sides (seams) on the outside. Sew around the top of the neck cowl- both the knit and sweater raw edges should be together.
Step #3: Pull the knit up and stuff it in to the sweater. Step #4: Sew around the other end of the cowl (which is the hem of the tank and the very top of the turtleneck.
Now you can enjoy the chilly weather with just one quick pull over your child's head. I used a Women's Large turtleneck and my son is almost four. It fits perfectly but will still stretch and fit in a couple of years. The added bonus is it fits around my head so I can use it as a neckwarmer or I could use it for an ear muff.