When we make our Sun Dresses, they are sized even sizes (just like girls here in the States) 2 through 14. The 12 and 14 sizes are of course meant for older girls, but because we don't add any to the width of the dress, we request that you put a 3-4" slit in the back (or sometime side) seam. It's REALLY easy, so please don't skip this step if the dress kit you get is longer than 32".
Here's a great tutorial showing step-by-step instructions. Try it! You'll be a pro in no time!! (Another good tutorial is at https://www.craftsy.com/post/how-to-sew-a-side-slit/ but you have to cut and paste that link for it to work.)