This study seeks to examine the efficacy and safety of the administration of autologous T cells that have been modified through the introduction of a chimeric antigen receptor (CAR) targeting the B cell surface antigen CD19 following administration of chemotherapy lymphodepletion regimen in children with relapsed or refractory acute lymphoblastic leukemia (ALL) or lymphoma. The overall goal of this study is to validate the safety profile of administration CD19-CAR T cells and describe the response rate in children with relapsed/refractory ALL or lymphoma.