S.W.
I haven't used the products yet, but I am sure it is in my future as my children are bi-racial. I would encourage her to love her hair the way it is to begin with, and once she knows how to care for it and style it that way, then she can try a different style. There is nothing wrong with wanting to try out different hair styles, the curly girls always want straight and the straight girls always want curly.
If her hair isn't going to get pin-straight with the product, then now is a good time for her to start to accept that. Just tell her how nice her hair is the way it looks, however it looks.
You can also straighten her hair with a flat iron or a round brush and blowdryer when you are drying it. That might not work on her hair without the relaxer, but with a good hair product and proper tools, you can most likely get her hair pin straight for a few days at a time. Just make sure to use lots of conditioner.
This is a tough one b/c you want her to know she is beautiful no matter what, and that what makes her beautiful is not her hair, but she's a girl, so she wants to feel pretty and there is nothing wrong with that. Good luck.