The present trial will be performed according to an open design to determine the maximum tolerable dose (MTD) by evaluation of dose-limiting toxicity (DLT) of volasertib in paediatric leukaemia and solid tumours in the age group 2 to less than 12 and 12 to less than 18 years. A further objective is to collect data on safety, tolerability, toxicity, efficacy (preliminary activity), pharmacokinetics and pharmacodynamics of volasertib in paediatric cancer patients