• Donors must be at least 18 years old
• Donors must be at least a single haplotype matched (3/6 or greater) family member
• Donors must be HIV negative
• Female donors of childbearing age must have a negative pregnancy test within 14 days before enrollment and not be breastfeeding with intent to breastfeed
• Donors must complete eligibility determination as per regulatory guidance
• Patients must be 21 years old or younger
• Patients must have relapsed and/or refractory CD19-positive leukemia confirmed within 2 months after any CD19 therapy
• Refractory disease includes primary refractory after at least 2 cycles of intensive chemotherapy or refractory after salvage therapy
• Relapsed disease includes second or greater relapse, relapse after allogeneic hematopoietic cell transplant (HCT), or first relapse requiring HCT but patient is ineligible or unsuitable
• Patients in Cohort A must have previously received HCT from the selected CAR T-cell donor
• Patients in Cohort B must not have received HCT from the selected CAR T-cell donor and must be unsuitable for autologous CD19-CAR T-cell therapy
• Detectable medullary CD19-positive leukemia
• Estimated life expectancy of at least 8 weeks
• Karnofsky or Lansky performance score of 50 or higher
• No CNS-3 disease or leukemia in CNS with neurological symptoms
• If prior allogeneic HCT, at least 3 months since transplant, recovered from therapy, no active GVHD within 2 months, and no donor lymphocyte infusion within 28 days before CAR T-cell infusion
• Adequate cardiac function (left ventricular ejection fraction 40% or greater or shortening fraction 25% or greater)
• EKG without significant arrhythmia
• Adequate renal function (creatinine clearance or GFR 50 ml/min/1.73 m2, or 40 ml/min/1.73 m2 if under 2 years old)
• Adequate pulmonary function (forced vital capacity 50% or greater predicted or pulse oximetry 92% or greater on room air if unable to perform test)
• Total bilirubin below 3 times upper normal limit except in Gilbert's syndrome
• ALT and AST below 5 times upper normal limit
• No history of HIV infection
• No severe uncontrolled bacterial, viral, or fungal infection
• Recovered from all grade III-IV non-hematologic acute toxicities from prior therapy
• Female patients of childbearing age must have a negative pregnancy test within 7 days before enrollment and not be breastfeeding
• Agreement to use birth control if sexually active until 6 months after CAR T infusion
• No history of hypersensitivity to murine protein-containing products
• Not receiving systemic steroids exceeding 0.5 mg/kg/day methylprednisolone within 7 days before CAR T infusion
• Not receiving systemic therapy within 14 days before CAR T infusion that interferes with CAR T activity
• Not receiving intrathecal chemotherapy within 7 days before CAR T infusion

History of severe allergic reactions to study medication Currently pregnant or breastfeeding Recent participation in another clinical trial within the last 30 days Presence of uncontrolled medical conditions that could affect safety