This pattern gives directions both in American and UK terminology.

Directions are always the same but the designer gives you an array of type of yarn and hook size to achieve different sizes, going from

Newborn (4 ply baby yarn, 3.5 mm /E4 hook)

Toddler 18 months- 2 years ( 8 ply, double knit or worsted weight yarn, 4 mm / G6 hook)