Showing Where Are the Children? by Mary Higgins Clark

Nancy Harmon has a new home, a loving husband and two beautiful children. The thing is, she's had all this before . . .
Seven years ago she escaped from a volatile marriage and the devastating deaths of her first two children. Now, she's trying to start afresh. The accusations. The newspaper stories. The blame. That's all behind her. Or so she thinks.
For someone has not forgotten. Somebody who is determined to bring the terror and the pain hurtling back. One cold morning, Nancy leaves her children to play outside - but when she returns, they have disappeared. With growing terror, she realises it has begun again . . .