If you have plenty of seam allowance on the sides, you could put the zipper on the left side of the dress, starting about an inch under the armpit, and going down as low as it takes for her to easily get out of the dress. I remember making a few dresses like this (i've been sewing for 50+ years).
Or take two strips of the fabric 2-1/2" wide, fold wrong sides together and press, giving you a 1-1/4" piece. Sew each strip down the back seam with a 1/4" seam, which will give you 1" extra down the back. Press the strips to the center so they overlap with one finished edge just covering the seam of the other side. If you need more than 1", just make your strips wider. I, too, prefer using snaps, and I would sew buttons on top of the snaps to hide them. I do like the idea of the ties, too, and you can add the ties when you sew on the fabric strips.