IMO..in my opinion :)
LOl. you are getting in uncharted territory for me, I would think if she is not done quite growing yet than her back is going to change more. And in my experience backs change with riding and work as well. I really don't know what to tell you, maybe find a saddle that will work for now and if it comes time that it doesn't fit maybe you can sell it and find one that works as you go. But I think the best would be to get with a good saddle maker in your area and her evaluated. He could be a better judge maybe find something that could work now or later with changing of pads?(I hate saying that)
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.