High white blood cells in children are usually caused by several factors. The first is due to bacterial infections, which often result in significantly higher white blood cell counts and generally higher C-reactive protein in children with bacterial infections. Secondly, it is caused by the stressful state of the child. When the child is vomiting or emotionally stressed, the white blood cells may also be high. Finally, children with acute leukemia may also have high white blood cells and may have a bone marrow aspiration to confirm the diagnosis.