M&S - they do sports bras and trainer bras that are non-padded in white. Perfect for a 10/11 yr old under a polo shirt. They come in A, AA, B, IIRC.
I think you have to measure under the boobies and then straight across and it's something to do with the difference in cm. I just guessed
But I might try to get her measured up this time as she is getting bigger.