Evaluation of Safety and Efficacy of Virus Specific T-Cell Administration in Pediatric Patients With Systemic Viral Infection Following Allogeneic Hematopoietic Stem Cell Transplantation.

The goal of this prospective clinical study is to evaluate the safety and efficacy of Multi-Virus Specific T cells (LB-DTK-MV) in pediatric patients with systemic viral infection, including CMV, EBV, and BKV, after allogeneic hematopoietic stem cell transplantation (allo-HSCT). The main questions it aims to answer are: * What is the maximum tolerated dose of LB-DTK-MV based on dose-limiting toxicity? * What treatment emergent adverse events occur within 14 days after the second infusion? * Is there a clinically significant reduction in CMV, EBV, and BKV viral loads within 14 days following the second infusion? * Is there a clinically significant improvement in clinical symptoms within 14 days following the second infusion? Participants will: * Receive a single intravenous infusion of LB-DTK-MV during the baseline visit (low dose: 1x10\^7/m\^2; high dose: 2x10\^7/m\^2). * Receive the second infusion of LB-DTK-MV intravenously at the same dose 14 days after the first infusion. * Attend weekly follow-up visits at the clinic for 6 months after the first infusion.