This crochet hat pattern is very simple and perfect for a beginner.The ribbed effect comes from working only in the back loop of each crochet stitch.
Also, the size can be worked as desired,for a baby and adult size.You just need to change the starting number of chain stitches(which forms the height) and the length(which forms the circumference) of the cap.
I made this cap for my 2 yr old Girl.

Materials Used:-

8 ply Acrylic Yarn 
5mm Crochet Hook