• PHASE 1: Patients between ≥ 1 year and ≤ 21 years of age at the time of study enrollment

• PHASE 1: Patients with recurrent/refractory solid tumors excluding primary central nervous system tumors

• PHASE 2: Patients with (FLC), which must include genomic confirmation of the DNAJB1:PRKACA fusion performed at a Clinical Laboratory Improvement Act (CLIA)-certified laboratory

• PHASE 1: Patients must have either measurable or evaluable disease

• PHASE 2: Patients must have measurable disease

• PHASE 1: Patient's current disease state must be one for which there is no known curative therapy or therapy proven to prolong survival with an acceptable quality of life

• PHASE 2: Patients must have FLC which is recurrent/refractory to at least one line of prior systemic therapy

• Patients with FLC that is unresectable at initial diagnosis but is not recurrent/refractory to at least one prior line of systemic therapy nor metastatic are NOT eligible for either phase 1 or phase 2

• Patients must have a performance status corresponding to Eastern Cooperative Oncology Group (ECOG) scores of 0, 1 or 2. Use Karnofsky for patients \> 16 years of age and Lansky for patients ≤ 16 years of age

• Patients must have fully recovered (grade \< 2) from the acute toxic effects of all prior anti-cancer therapy and must meet the following minimum duration from prior anti-cancer directed therapy prior to enrollment. If after the required timeframe, the numerical eligibility criteria are met, eg, blood count criteria, the patient is considered to have recovered adequately

‣ Cytotoxic chemotherapy or other anti-cancer agents known to be myelosuppressive: See Developmental Therapeutics (DVL) homepage on the Children's Oncology Group (COG) members site for commercial and investigational agent classifications. For agents not listed, the duration of this interval must be discussed with the study chair and the study-assigned research coordinator prior to enrollment

• Solid tumor patients: ≥ 21 days after the last dose of myelosuppressive chemotherapy (42 days if prior nitrosourea). Please refer to the table of myelosuppressive/anticancer agents on the COG website

⁃ Anti-cancer agents not known to be myelosuppressive (eg, not associated with reduced platelet or absolute neutrophil count \[ANC\] counts): ≥ 7 days after the last dose of agent. See the DVL homepage on the COG Members site for commercial and investigational agent classifications. For agents not listed, the duration of this interval must be discussed with the study chair and the study-assigned research coordinator prior to enrollment

⁃ Antibodies: ≥ 21 days must have elapsed from infusion of last dose of antibody, and toxicity related to prior antibody therapy must be recovered to grade ≤ 1

⁃ Corticosteroids: If used to modify immune adverse events related to prior therapy, ≥ 14 days must have elapsed since last dose of corticosteroid

⁃ Hematopoietic growth factors: ≥ 14 days after the last dose of a long-acting growth factor (eg, pegfilgrastim) or 7 days for short acting growth factor. For agents that have known adverse events occurring beyond 7 days after administration, this period must be extended beyond the time during which adverse events are known to occur

⁃ Interleukins, interferons and cytokines (other than hematopoietic growth factors): ≥ 21 days after the completion of interleukins, interferon or cytokines (other than hematopoietic growth factors)

⁃ Stem cell Infusions (with or without total body irradiation \[TBI\]):

• Allogeneic (non-autologous) bone marrow or stem cell transplant, or any stem cell infusion including donor lymphocyte infusion (DLI) or boost infusion: ≥ 84 days after infusion and no evidence of graft versus host disease (GVHD)

∙ Autologous stem cell infusion including boost infusion: ≥ 30 days

⁃ Cellular therapy: ≥ 30 days after the completion of any type of cellular therapy (eg, modified T cells, natural killer \[NK\] cells, dendritic cells, etc.)

⁃ Radiotherapy (XRT)/external beam irradiation including protons: ≥ 14 days after local XRT; ≥ 150 days after TBI, craniospinal XRT or if radiation to ≥ 50% of the pelvis; ≥ 42 days if other substantial bone marrow (BM) radiation

⁃ Radiopharmaceutical therapy (eg, radiolabeled antibody, lobenguane I-131 \[131I-MIBG\]): ≥ 42 days after systemically administered radiopharmaceutical therapy

⁃ Patients must not have received prior Bcl-xL specific therapy (e.g. navitoclax, DT2216). Prior therapy with irinotecan or other topoisomerase 1 inhibitors and/or other BH3 mimetics which are not Bcl-xL selective (e.g. venetoclax) are acceptable

• For patients with solid tumors without known bone marrow involvement: Peripheral absolute neutrophil count (ANC) ≥ 1000/µL

• For patients with solid tumors without known bone marrow involvement: Platelet count ≥ 100,000/µL (transfusion independent, defined as not receiving platelet transfusions for at least 7 days prior to enrollment)

• Patients with known bone marrow metastatic disease will be eligible for study provided they meet the blood counts (may receive transfusions provided they are not known to be refractory to red cell or platelet transfusions). These patients will not be evaluable for hematologic toxicity. At least 5 of every cohort of 6 patients must be evaluable for hematologic toxicity for the dose-escalation part of the study. If dose-limiting hematologic toxicity is observed, all subsequent patients enrolled must be evaluable for hematologic toxicity

• A creatinine based on age/gender as follows:

‣ 1 to \< 2 years: Maximum serum creatinine (mg/dL) 0.6 (male), 0.6 (female)

• OR a 24 hour urine creatinine clearance ≥ 70 mL/min/1.73 m\^2

∙ OR a glomerular filtration rate (GFR) ≥ 70 mL/min/1.73 m\^2. GFR must be performed using direct measurement with a nuclear blood sampling method OR direct small molecule clearance method (iothalamate or other molecule per institutional standard). Note: Estimated GFR (eGFR) from serum creatinine, cystatin C or other estimates are not acceptable for determining eligibility

• Patients with solid tumors: Bilirubin (sum of conjugated + unconjugated or total) ≤ 1.5 x upper limit of normal (ULN) for age

• Patients with solid tumors: Alanine aminotransferase (ALT) ≤ 3 x ULN, unless attributed to tumor involvement then ALT ≤ 5 x ULN

‣ Note: For the purposes of this study the ULN for ALT is defined as 45 U/L

• Patients with solid tumors: Aspartate aminotransferase (AST) ≤ 3 x ULN, unless attributed to tumor involvement then AST ≤ 5 x ULN

‣ Note: For the purposes of this study the ULN for AST is defined as 50 U/L

• Patients with solid tumors: Albumin ≥ 2 g/dL

• Patients with solid tumors: International normalized ratio (INR) ≤ 2.5

• Patients with seizure disorder may be enrolled if on anticonvulsants and well controlled as evidenced by no increase in seizure frequency in the prior 7 days. If needed, evaluate use of enzyme-inducing anticonvulsants

• Nervous system disorders (Common Terminology Criteria for Adverse Events \[CTCAE\] version \[v\]5) resulting from prior therapy must be ≤ grade 2, with the exception of decreased tendon reflex (DTR). Patient with any grade of tendon reflex decrease are eligible