I keep mentioning this book without reviewing it, so I'm going to remedy that now. The Godsend was published in 1976, but I've seen a few copies around at used book sales and if you're interested you could probably find a copy on eBay or BookMooch.
As I said in my review of Orphan, the plot of that film and this book are very similar. Alan and Kate have the perfect family with four young children. Until they take in a pregnant drifter and she gives birth, leaving them with the baby girl. The couple adopts her, names her Bonnie and loves her just as much as their other children. But as she grows older, the other children begin to die in horrible ways. Alan blames Bonnie, but Kate is blinded by her love for the child and can't see that she's evil. Alan must get Kate to see the truth about Bonnie before she kills all their children.
The Godsend is slow to start, but once Bonnie enters it picks up the pace. And then it was hard to put down. I wanted to keep reading to know what Bonnie would do next and when Alan and Kate would finally figure out that it's her hurting the other children.
I had a love/hate relationship with the characters. I loved Alan and Kate, but hated how dumb they were at times. At least Alan eventually figured it out. Sometimes I just wanted to slap Kate for being such a moron. All of the kids were cute and didn't deserve what Bonnie dished out.
I don't want to give anything away, but the ending is bleak and depressing. But it matched the tone of the book. It would've been odd to suddenly wrap everything up with a happy ending.
If you love killer kids, I highly recommend The Godsend.