What mechanism has gone awry when cancer develops?

All cancers are characterized by uncontrollable cell growth and division, and when untreated, are fatal. Cancers that occur in childhood are usually more malignant than those developed later in life. Why do you suppose there is a higher degree of malignancy in childhood? Why are childhood cancers more likely to have a hereditary predisposition than adult onset cancers? What mechanism has gone awry when cancer develops?