I'm working on dresses for my three grand-girls (6, 8, and 9 years) and I chose to use a cotton/acrylic blend because of how 100% cotton stretches and loses shape. I went with Filatura Lanarota Cool Cotton because 1) it is a blend that has great reviews on Ravelry and 2) it was on sale in acceptable colors on www.smileysyarns.com
Be aware that the pics of colors at on-line sites may be a little off so you may want to go to Ravelry and check out the yarn color in other people's stashes to get a better idea of the actual color. For example, color #54 in the pic on smileysyarns.com looks purple but it is actually more magenta (purpley-red-violet) than it looks. I checked Ravelry before I bought it to make sure and when I received it, it really was a deep magenta (which is what the 6 y/o wanted).