The nappy/diaper cover is created in one size for a newborn (or up to a few months old depending on birth weight). An exact gauge isn’t necessary.

To knit the diaper and crown I used just over 50g.

The crown can be knitted in any size as it’s knitted sideways and you can add additional peaks - I have knitted a one-year old for a boy’s birthday party with 9 peaks whereas a newborn is knitted with 6.

Lovecrafts: https://www.lovecrafts.com/en-gb/p/knitted-crown-and-napp...