Yes, I've read them. I have very mixed feelings about them. Are they appropriate for children? No. Stephenie Meyer writes about seventeen year olds, and thus these books might be intended for seventeen year olds but that doesn't mean that these books are appropriate for young adults. I mean just because Edward and Bella don't do anything but kiss and clutch, they still share the same bed night after night, all the while keeping her father in the dark. Some have told me, "It's completely innocent." How can that be true when Bella herself says she loses control everytime their lips touch?
Let me backtrack...Bella moves to Forks, Washington after living in Phoenix, Arizona most of her life. Her first day at a new school, she notices the beautiful but distant Cullens family. She sits next to Edward Cullens in science and senses that he hates her. She also notices his black irises, (in his eyes, not flowers) and his extremely cold skin. Edward misses the next few days of school. Much of the story is told in the mind of Bella, not much action or dialogue compared to the inner workings of Bella's psyche.
Speeding things up, Bella falls for Edward, who now has topaz irises. She is warned to stay away from him and his family, by Billy Black who is fifteen year old Jacob's grandfather. Jacob tells Bella about his tribe's legends and why the Cullens aren't allowed on LaPush Reservation land. He refers to the Cullens as the "cold ones" and his own tribe as the protectors. Bella clues in and does some internet research on vampires.
She asks Edward about his "family" and learns that the Cullens are "vegetarian" vampires. They have chosen not to feed on human prey, rather they hunt wild game: grizzlies, mountain lions, elk. Vampires are created, not born (by surviving a vampire bite). Carlisle, the patriarch of this group, or "coven" of vampires was created by an attacking vampire in the 1600s. He has never taken a human life, and rather uses his superior senses to heal, working as a physician in the local hospital. The other vampires, Esme, Edward, Jasper, Alice, Emmet, and Rosalie were either created by Carlisle, because they were on the brink of human death, or created by others, led to Forks and willingly joined the Cullen way of life.
Now there are still plenty of other vampires, who murder innocent humans for food, and sometimes they visit the Cullens. But the Cullens respectfully ask them not to "hunt" in their area, so usually they go out of state. Jacob, and other young male members of his tribe, the Quileute tribe, have spontaneously starting changing into werewolves. They phase back and forth between human and wolf form, and strive to hunt vampires. We find out in the 4th book that they aren't actually werewolves but shape-shifters.
So that's the back story. I have not read the third installment, but the first two books are steeped with what Publishers' Weekly calls "sexual tension." I hate to criticize anyone's writing skill, but when Bella asks Edward in New Moon, "Do you want me for my body or my blood?" I actually laughed out loud. As far as an overall theme of good vs. evil, the plot of the book appears to be keep Bella alive, safe from friends, good vampires and bad ones, so that she and Edward can test their willpower while they fool around with each other's lips and bodies. There is some discussion of souls, heaven and hell and whether or not Edward believes he has a soul, which make these books slightly better than paperback horror/romances. Granted, in Breaking Dawn, Bella and Edward do wait until their wedding night, and she does choose to keep her half vampire baby against everyone's advice. However, her reckless moral behavior prevents her from becoming a true heroine. (I did not read the third book, Eclipse, as I do not want to spend my money on these, and the waiting list at the library is very long.)