Felting is a fun knitting adventure. Most knitting patterns call for worsted or bulky yarn, but felting with thinner yarns creates a finished fabric that I think is more attractive and wearable. This hat is made using DK weight yarn, and following this pattern with sport weight yarn will make a good hat for a child.
When felting, you'll have the best results if you make "before" and "after" swatches that you can compare so that you can extrapolate exactly how much your yarn will felt. Felting can be a bit unpredictable, because even dye lot differences can change how much and how quickly your project will shrink. That said, if you use stitch numbers appropriate for the normal gauge of your yarn, but work with a larger needle and add some extra length, things will usually turn out all right.