There is no specific gauge for this pattern as long as the leg warmer fits your little one. My son is 13 months and the leg warmers fit very nicely and he can still grow up more, there is room for that :). Each child is different, specifically at this age. If you need to decrease or increase the number of stitches, make sure you cast on a multiple of 4 plus one stitches for the Coin stitch pattern.
To help, I knit one repeat unit of the Coin Stitch used for this pattern and it is about 2 cm long.