Can you even buy a phone for $50?????? Maybe keep the money, take her to a phone shop and have a look with her. She'll see that the phones are much more expensive than $50. (Unless I'm wrong). Then suggest to her she might like something else.
You can get really cheap ones for under that these days. But they do nothing. Which is probably a really good thing for someone as young as she is!

I wouldn't do it. Stand your ground your the one that will have to deal with her crying over not having any credit.