I don't know of any *text* books that would be appropriate, but I've learned far more about improving my photographic "eye" by looking at excellent photos than by reading. As long as she has someone (who understands the technical basics of photography) to explain how to fully use the features of whatever camera she is currently using, I'd recommend getting her some big picture books of photos both by some of the great photographers of the past and by some excellent photographers of the present whose work would particularly appeal to children. Someone could suggest "tasks" or "games" such as trying to imitate the style of some particular photographer.
Lisa