It's difficult for those of us who are parents to give advice to our adult children; they have their own lives now, and they may resent us trying to tell them what to do. Nevertheless, sometimes we need to make the effort -- and this may be one of those times.
You love your granddaughter -- and because you do, you don't want her to be led astray or tempted to follow ways that will only harm her. Every day our young people are on the receiving end of a barrage of messages urging them to live in a certain way -- but most of those messages are false. Helping them see this is one of the hardest challenges any parent faces today. We are like the people of Isaiah's day: "O my people, your guides lead you astray" (Isaiah 3:12).
Pray for your granddaughter, and ask God to give you wisdom to know how to express your concern -- both to her and to her parents. Instead of lecturing them, you might simply ask her parents if they're aware of what she's reading, and if they've given any thought to its impact. The Bible says, "Whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable... think about such things" (Philippians 4:8).
Most of all, ask God to help you point your granddaughter (and her parents) to Jesus Christ. He alone gives lasting happiness and meaning to our lives.