I would highly suggest checking out the Disboutiquers thread on the family board if you might try sewing. And a few suggestion is you want to do it as easily as possible. One of my favorites is the tshirt dress. Take a nice tshirt, cut it to waist or empire length on your daughter. Then take a rectangle of fabric, gather to the same width as the hem of your cut shirt and sew it one. Just hem the fabric at the bottom. You can use a Disney shirt, Disney fabric, or plain shirt or fabric. Any combination you and your daughters like. Your can always sew fancy trim on the bottom or sleeve hems too if you want. These are very comfortable for the girls, and can still be a 'princess' dress.
If you are more ambitious, and peasant dress done in the correct colors or fabrics will look very nice too. And still be quite comfortable. Both can be layered with long sleeved shirts or leggings. For that matter, both can be made with long sleeves. Check in with the Disboutiquer's if you considering making something, it's a very helpful group who can give you far more suggestions then I can. If all else fails, I second the idea of Etsy-you have to pay, but people do some beautiful work.
Mary