This pattern is called a Cap in the pattern instructions and a Helmet in the materials list.

Measurements
To fit toddler from 1 year to 2 years old

Each pattern has a different set of recommended yarns. The following is from the Peter Green version.

Materials
Total yarn needed for the complete set of garments
DOUBLE KNITTING
Bébé Quickerknit
15, 17 balls
Triacetate Nylon
19, 22 balls
Grenlon
19, 21 balls
Superwash
18, 20 balls
Super Cosiwear or Pamlon
16, 18 balls
1 button for helmet.