A Study of Venetoclax in Combination With Navitoclax and Chemotherapy in Subjects With Relapsed/Refractory Acute Lymphoblastic Leukemia or Relapsed/Refractory Lymphoblastic Lymphoma

* Must have relapsed or refractory acute lymphoblastic leukemia (ALL) or relapsed or refractory lymphoblastic lymphoma (LL). Refractory is defined as persistent disease after at least 2 courses of chemotherapy. * Participants with ALL with Philadelphia chromosome or with an ABL class targetable fusion are eligible. * Participants with LL must have radiographic evidence of disease * Participants \<= 18 years of age who do not have a standard of care treatment option available. * Must weigh greater than or equal to 20 kg. * Must be able to swallow pills. * Must have adequate hepatic and kidney function. * Must have adequate performance status: * Participants less than or equal to 16 years of age: Lansky greater than or equal to 50 * Participants greater than 16 years of age: Karnofsky greater than or equal to 50 or Eastern Cooperative Oncology Group (ECOG) less than 3.

* Participant has central nervous system (CNS) disease with cranial involvement that requires radiation. * Participants who are less than 100 days post-transplant, or greater than 100 days post-transplant with active graft versus host disease (GVHD), or are still continuing post-transplant immunosuppressant therapy within 7 days prior to the first dose of study drug. * Participants who have received any of the following prior to the first dose of study drug: * Inotuzumab within 30 days (if participant received inotuzumab \> 30 days prior to Day 1, must have ALT, AST and bilirubin \< ULN). * A biologic agent (i.e., monoclonal antibodies) for anti-neoplastic intent within 30 days * CAR-T infusion or other cellular therapy within 30 days * Any anti-cancer therapy including blinatumomab, chemotherapy, radiation therapy targeted small molecule agents or investigational agents within 14 days, or 5 half-lives, whichever is shorter * Exception: Philadelphia Chromosome (Ph)+ ALL subjects on TKIs at Screening may enroll and remain on Tyrosine Kinase Inhibitor (TKI) therapy to control disease. Participants on venetoclax at screening may enroll and remain on venetoclax. * Steroid therapy for anti-neoplastic intent within 5 days * Hydroxyurea that is ongoing (hydroxyurea is permitted up to the first dose) * A strong or moderate CYP3A inhibitor or inducer within 7 days * Aspirin within 7 days, or 5 half-lives, whichever is longer * An excluded antiplatelet/anticoagulant drug or a herbal supplement that affects platelet function within 7 days, or 5 half-lives, whichever is longer * Participants with malabsorption syndrome or any other condition that precludes enteral administration.