Using different yarn and crochet hook will influence the size of the hat as well as your stitches.
For a child beanie use US H/5mm for the brim and US I/5.5mm for the body using the same stitches unless you want an even smaller brim.

Gauge using the back loop method

Worsted weight yarn (4) Red Heart or Caron (pound) Yarn that holds the structure. Unless you want to use soft yarn.

1 skein
If you want to make the brim a different color you will need 18 grams.