Twirl Skirt from Indietutes - This tutorial is very easy to follow. All you need is a few strips of fabric!
Twirly Skirt from House on Hill Road - Another simple tutorial using larger fabric swatches.
Matching patchwork twirly skirts. Follow the easy steps for a girl-sized skirt at Indietutes and for a doll-sized skirt at Twirls for Girls. It can fit an American Girl or similarly sized doll.
More Easy Skirts and Dresses
Skirts from dishtowels from Grace Violet.
Solar dress - Super easy, super adjustable fluid dress from Indietutes. The delightful model is wearing the dress while coloring the pattern for the dress.
Recycled sweater gore skirt from Martha Stewart - Make one for a little girl and follow the same instructions to make a pattern for a doll
For general resources for sewing for kids check out the tutorials at Whip up including this kimono wrap.
Or try this highly rated book "Sewing Clothes Kids Love."