Although the pathogenesis of Coronavirus Disease 2019 (COVID-19) is not fully described, the interaction between the SARS-CoV-2 and the immune system is so complicated, and lymphopenia, hyper-inflammatory responses, and cytokines play an essential role in the pathology of COVID-19. Among the laboratory features of COVID-19, some hematological abnormalities have been described; however, no studies about lymphocyte subset analyses have been reported in children. Therefore, this study aimed to explain the characteristics of lymphocyte subsets in pediatrics with mild or severe COVID-19. Subjects of the study were children with confirmed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pneumonia by real-time RT-PCR admitted to the Children’s Medical Center, affiliated to the Tehran University of Medical Sciences. Complete blood count and lymphocyte subpopulations were studied for all patients. The study population included 55 hospitalized patients with confirmed SARS-CoV-2 infection (34 patients (62%) with mild and 21 patients (38%) with severe disease). Lymphocyte counts were significantly lower in patients with severe disease (mean± sd 1.6± 0.9 in the severe group vs. 2.3± 2.2 in the mild group). Compared with mild pneumonia, children with severe pneumonia had a higher count of CD8+ lymphocyte, while a lower percentage of CD4+ lymphocyte was observed; however, the differences were not significant. Interestingly, severe group had a significantly lower CD4+/CD8+ T cell ratio compared to the mild group (1.1 ± 0.47 vs. 1.4 ± 0.8, p-value: 0.039). CD4+/CD8+ less than 2, 1.5, and 1 was found in 48 (87%), 40 (73%), and 19 cases (35%). Among 7 cases (13%) who died, 100% had CD4+/CD8+ ratio<2, 86% had CD4+/CD8+ ratio<1.5, and 29% had CD4+/CD8 + ratio<1. Lymphocyte counts and lymphocyte subset (CD4+ /CD8+ T cell ratio) reflect the disease severity, which can be considered as a potential predictor for disease severity of COVID-19 in children.