From designer:
The pattern is for kids 4-10 years-old. The ribbed fabric is clingy and stretchy at the same time which is perfect to fit a wide range of foot sizes. The pattern is simple and a good way to start knitting socks if you haven’t tried before. These will go quickly and will teach you the basics of sock knitting in no time flat.

Correction:
Gusset:
Rnd 1:
Needle 1: knit
Needle 2: continue in rib pattern (k3, p1)
Needle 3: knit