B7-H3.CD28Z.CART in Solid Tumors

The goal of this research study is to test if a new cell therapy (B7-H3.CD28Z.CART / B7-H3 CAR T cells) is safe and effective in treating children and young adults with solid cancers whose tumors have returned or stopped responding to standard treatments (relapsed or refractory) and have been identified with a B7-H3 marker. The names of the treatment interventions used in this study are: * B7-H3.CD28Z.CART / B7-H3 CAR T cells * Fludarabine * Cyclophosphamide

Inclusion Criteria: Eligibility Criteria for Prescreening Purpose of prescreening is to establish B7-H3 expression by IHC performed at Boston Children's Hospital. This can be performed at any time prior to completing the protocol screening process. Participants who meet the following criteria, will be offered participation in the full screening process and protocol enrollment, if eligible: * Participants must have histologically confirmed diagnosis of a solid tumor that is relapsed or refractory for which standard curative measures do not exist or are no longer effective. * Participant must have adequate pre-trial tumor material available to determine B7-H3 status. Tumor tissue from the most recent resection or biopsy of recurrent disease is preferred. If unavailable, tumor tissue from prior recurrences or from the time of initial diagnosis is acceptable. * Age \>=9 months and \100 by immunohistochemistry (IHC) is required by IHC performed at Boston Children's Hospital. Participants may choose to enroll on the prescreening portion, which allows for assessment of B7-H3 expression only, prior to enrollment on the full clinical trial as described in Section 3.1. * Age \>=12 months and \50% marrow). * At least 2 weeks since any prior myelosuppressive chemotherapy * At least 28 days from other investigational antineoplastic or disease-directed agents * At least 7 days from most recent myeloid growth factor, at least 14 days must have elapsed after receipt of pegfilgrastim. * At least 7 days from prior biologic antineoplastics, tyrosine kinase inhibitor, targeted agent or metronomic non-myelosuppressive chemotherapy. * At least 21 days or 5 half-lives, whichever is shorter, post any treatment with monoclonal antibodies (including checkpoint inhibitors and bevacizumab) * At least 7 days from dinutuximab treatment * At least 8 weeks from prior cellular therapy or vaccine therapy with recovery of associated toxicities. If prior CAR T cells, need documented lack of persistence of prior product. * At least 6 weeks post 131I-MIBG therapy or other radioisotope therapy * At least 6 weeks post autologous stem cell therapy infusion following myeloablative conditioning * Participants can be eligible after autologous stem cell infusion without myelosuppressive therapy at any time as long as other criteria are met. * At least 12 weeks post allogeneic stem cell transplant with no evidence of GVHD or ongoing toxicities. * Steroid use: Corticosteroids at or below physiologic doses (replacement therapy for management of pituitary/adrenal insufficiency) is allowed and/or topical administration (e.g. inhaled or dermatologic) is allowed. Hydrocortisone for blood product premedication is allowed. * Participants must have normal marrow function as defined below: Values must be without transfusions or platelet growth factor within 7 days. Participants with known marrow involvement are exempt from these requirements. * hemoglobin ≥7.0g/dL * absolute neutrophil count ≥750/mcL * platelets ≥75,000/mcL * Adequate renal function defined as creatinine below normal limits for age OR creatinine clearance (as estimated by Cockcroft Gault Equation for participants ≥ 18yo and Bedside Schwartz for participants \