Knowing that she has depression may also encourage her to take treatment seriously and to work toward recovery, as she would if she had any other illness.
These are all factors to consider when weighing your decision. Again, you know your child best.
If you choose to discuss her depression with her, do so on an age-appropriate level and keep it as positive as possible. That last part may sound like an oxymoron. Depression...positive? The truth is that there are many people who have struggled with depression and have gone on to live happy and successful lives. Try to find such success stories in movies, books and even in reports of personal experiences of everyday people. You can start right here on About.com:
G.S. Truant, M.D. J.G. Lohrenz, M.D. "Basic Principles of Psychotherapy I. Introduction, Basic Goals, and the Therapeutic Relationship." American Journal of Psychotherapy. Winter 1993 47(1): 8-19.