600 or 500 spine, depending on the length of the shaft and the point weight. One of the things I looked at was the availability of broadhead weights (assuming she is going to hunt). If she is not hunting, mute point. I wouldn't put a 65 grain practice point on an arrow if all I could get was 85 grain broadheads.
I would go with the stiffer shafts if she may be increasing her draw weight and cut them a little long to take into account her draw length also increasing as she grows. It is much easier to get good flight from a shaft that is on the stiff side than one that is too weak.