RecruitingPhase 1Drug trialA Phase I Study of Hyper-CVAD In Combination With Venetoclax In Pediatric Patients With Relapsed or Refractory Acute Leukemias That Are of the Lymphoid Lineage Including Bi-Phenotypic or Undifferentiated LeukemiasTo find the recommended dose of hyper-CVAD in combination with venetoclax that can be given to participants with relapsed or refractory leukemia.