Commonest haematological malignancy in children is:

Acute lymphoblastic leukemia (ALL) is the most common malignancy of childhood, accounting for about 25% of all cancer diagnoses in patients younger than age 15 years. The peak age at onset is 4 years; 85% of patients are diagnosed between ages 2 and 10 years. Leukemia is defined by the presence of more than 25% malignant hematopoietic cells (blasts) in the bone marrow aspirate. Ref: Graham D.K., Quinones R.R., Keating A.K., Maloney K., Foreman N.K., Giller R.H., Greffe B.S. (2012). Chapter 31. Neoplastic Disease. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e.