From the pattern

The mock cable pattern goes all the way down to the heel, and all the way to the toe.

The mock cable pattern is very stretchy, so the socks can be made to fit a variety of sizes simply by altering the length of the sole. If it is needed to change the length of the heel flap, be sure to adjust the number of stitches picked up on each side accordingly. Please note, however, that shortening the heel flap is not recommended as it will be difficult to work the gusset decreases with fewer stitches.”

Measurements
Unstretched, the circumference measured on the inside of the cuff is approximately 11cm. It stretches up to about 18cm.
The slim fit version would fit a child with a leg circumference of 11cm - 18cm (18cm will be snug).