National Statistics on Missing Children
Every ten years or so the Department of Justice undertakes a study of missing children, the National Incidence Studies of Missing, Abducted, Runaway and Throwaway Children. NISMART II, the most recent, was released in October 2003.
These studies are in PDF format. You may need to download the free Adobe Reader to view them.
Missing Child Homicide Statistics
The other study that is often quoted by media and by professionals in the missing child field was conducted by the Attorney General of Washington State in 1997.
The findings were that among abducted children who were murdered, "in 74 percent of the cases the victims were dead within three hours after abduction." What is not generally reported is the fact that this statistic refers to a very small group of children abducted by violent or predatory kidnappers (approximately 1 in every 10,000 reports of a missing child).
So yes, it is important to find the child quickly, in case the kidnapping is one of that small group. But it is incorrect to assume that 74% of kidnapped children are at risk of being killed if not found within 3 hours.
You can download the Executive Summary of that report here in PDF format. You can also obtain more information about the study by visiting the Attorney General's website.