many LQS have a tray of mixed sizes of thimbles from very smallest to largest.
Do you know what size she needs. Why don't you try to make a circle of the size of her finger tip and post it here in case any of us has the size you eed. My mother had very small hands and I do have boxes of thimbles I use for classes and a good LQS locally
So small an be a numbr of sizes and without knowing just how small it is hard to give suggestions.