Oral mucositis (OM) is a frequent, debilitating complication in pediatric oncology that impairs quality of life, nutrition, hydration, and treatment adherence. This randomized, prospective, single blind trial in Mexico will evaluate photobiomodulation (PBM) versus a conventional bioadhesive gel for prevention and treatment of antineoplastic therapy-induced OM in children aged 4-17 with leukemia, lymphoma, or head and neck tumors. A total of 49 participants will be enrolled. The study has two components: (1) Treatment - parallel comparison of PBM versus bioadhesive gel for established OM; (2) Prevention - crossover design in which patients receive both interventions across successive chemotherapy cycles. PBM will be delivered with a 660 nm device, 40 mW, 10 J/cm². The primary outcome is OM grade by the WHO scale assessed on days 7, 11, 14, and 21. Expected results include reduced OM incidence, severity, duration, and pain with favorable safety and tolerability, supporting standardized PBM protocols in pediatric oncology in Mexico.