Blue heaven / C.J. Box.

A twelve-year-old girl and her younger brother go on the run in the woods of North Idaho, pursued by four men they have just watched commit murder--four men who know exactly who William and Annie are, and who know exactly where their desperate mother is waiting for news of her children's fate. Retired cops from Los Angeles, the killers easily persuade the inexperienced sheriff to let them lead the search for the missing children. William and Annie's unexpected savior comes in the form of an old-school rancher teetering on the brink of foreclosure.--From publisher description.