You, on the other hand, call yourself a Christian grandmother. A grandmother you might be, but Christian? That's questionable. Jesus Christ Himself said it is better to put a millstone around your neck and toss yourself into the sea than harm a child. And that is what this 15-year-old is.
Further, for you to find fault in the girl's parents to do what was legally viable to protect their daughter instead of giving her carte blanche says much more about you than the parents. A parent wants to protect their child, not toss them to the wolves. That's why we are having the problems we are with today's kids in the first place: We don't discipline. We don't protect.
For you to rationalize or defend any of this is despicable. How dare you.
In the society and culture Jesus grew up in you were an adult at 13.
So...you can read the hearts and minds of your fellow human beings. Impressive. Have you told that to God, yet...that you are happy to do his job?