This is a Phase 1b study of EPZ-5676 in pediatric patients. The study will have two phases. The first phase will assess escalating doses of EPZ-5676 in order to determine the maximally tolerated dose (MTD) or recommended phase 2 dose (RP2D) of EPZ-5676 as a 28-day continuous IV infusion. Once the MTD and/or RP2D is established, a second phase of the study will further evaluate the safety of EPZ-5676 and assess the anti-leukemia activity.