The quilt is adorable!! I might not touch it.
But - if I were to do anything with it - this is what I would probably do. When I have had this problem and want to fix it, I run a straight stitch down the edge just inside the seam allowance before I put on the binding (sometimes just the stitch alone is enough to ease in the wave). I use a stitch just a tiny bit longer than what I would usually use and loosen the tension just a bit as well. I then measure the center on the quilt and tug on one of my stitch threads (usually it will be the bottom thread) essentially "gathering" in the edge but not tightly enough to have any real gathers or pleats. It is kind of like setting in a sleeve - all you want is to ease in the extra fabric. I have also used this method if I have an unruly block that is just a bit off and I need something in it to match up perfectly to another block.