Hi C.!
I can only share what happened to me as a young girl, and I clearly remember although it was over 20 years ago! I developed on one side before the other, noticably! My mom rushed me to the doctor in a panic only to find out it was normal (puberty), making me feel like something was wrong with me! She made this HUGE ordeal out of it...rushing me to buy bras etc., and I just remember wishing it would go away!
My only advice would be just to talk to her, or as others have suggested, buy her a book that can help her understand. Reassure her it's completely normal and ok, and a wonderful and beautiful thing! If the need to buy bras comes up, which it inevitably will...make it a girl's day out...lunch, bra shopping, maybe a pedicure, etc!
Best of luck to you and your little girl!