If you have a sewing machine and can sew a mostly straight line, you can do this.
This fabric (and others like it) comes pre-made by the yard. The top is pre-smocked, so all you have to do is measure the chest width of your child, cut it, and sew a seam down the back. You could leave it strapless for a cute swimsuit cover up, or pick out some cute ribbon and sew straps. Just measure on your daughter and pin them where they belong and sew. It's literally the easiest thing I've ever made. Finishing something that is wearable, no matter how easy, is quite satisfying as well!
You can get the fabric and ribbon at Hobby Lobby with a coupon (make sure you have the app) or when it's on sale, which is often. They sell it at other fabric stores as well. Happy dress making!
My daughter, above, wore this to school today with leggings and a little tank top underneath.