Super simple and super fast and VERY fun. The hat pattern is below.

Yarn: Cascade 220 Superwash colors Chocolate, Magenta, and Pink Ice are shown, but you can use any three colors of a worsted weight yarn that will get the correct gauge.
Needles: For me, a Clover 48” circular, US 4, but you should swatch and use whatever needles get you the proper gauge!
Gauge: 5 sts/inch in stockinette stitch
Notions: Clover (or other, this just reflects my bias toward a favored multi-tasker) locking ring stitch markers (8 of them), tapestry needle, imagination.
Finished size: about 16 inches in circumference.

The finished hat will fit a toddler, from about age 2 to about age 5, depending on the size of the kid in question. It may fit a 11 year old, if a bit snugly, as mine did (I just slapped it on one to see). It’s snug on me, but I’d wear it, and my head is about 20 inches around. PLEASE feel free to adapt, modify, make bigger or smaller by adjusting gauge, or number of rounds knit, or casting on an extra inch or two’s worth of stitches, or what-have you. This is a really basic pattern, and it assumes the knitter has a basic skill set of simple decreases and circular knitting techniques, or knows where to get information on the same.