I hated hair stuff as a kid because my mother insisted it be combed which took valuable time away from playing. Didn't actually start enjoying looking after my hair until my 20's, before then it was just stuff on my head, still is sometimes
Your niece has a good starting point since she already knows what she wants; long hair
Tell her she has beautiful hair and it can grow as long as she wants it to
All she has to do is look after it
And then give her the means and the tools and any physical help she may need looking after it
Some girls do length checks, make a note of how long the hair is now and then check however many months later, it feels good to see results
There are so many women out there who decided to put in the effort and its paid off. Show her pictures from this thread:
http://www.thecoli.com/threads/natural-hair-appreciation.208644/