Pediatric Cancer: is cancer in a child. In the United States, an arbitrarily adopted standard of the ages used are 0-14 years inclusive, that is, up to 14 years 11.9 months of age. However, the definition of childhood cancer sometimes includes adolescents between 15-19 years old. Leukemias, which are cancers of the bone marrow and blood, are the most common childhood cancers. They account for about 30% of all cancers in children. The most common types in children are acute lymphocytic leukemia (ALL) and acute myelogenous leukemia (AML).