Causes of significantly high white blood cell counts in children include the following: 1. Conditions caused by a child suffering from a bacterial infection. This is the most common cause of high white blood cell count in children. 2. Due to stress in children. This is the most common cause of high white blood cell count in children. 3. If a child is suffering from leukemia, the white blood cell count will also be significantly high and a bone marrow aspiration is needed to confirm the diagnosis.