Background: COVID-19 is usually milder in children and adolescents, leading to lower hospital admission rates than adults. This study evaluated clinical manifestations in children (< 10 years) and adolescents (10 to < 18 years) with COVID-19 admitted to a tertiary municipal hospital in Rio de Janeiro (Brazil) during the first (February to November 2020) and second pandemic waves (November 2020 to April 2021). Methods: this retrospective observational study considered patients in the pediatric age group (<18 years old) with confirmed diagnosis of COVID-19 using RT-PCR. Descriptive and bivariate analysis were performed assuming a p-value<0.05 level of significance for all analyses. Results: among the 34 included patients (50% boys; 73.5% children), the most prevalent symptom was fever (88.2%), followed by asthenia (85.3%), and cough associated with dyspnea (50%); 29.4% were admitted to the ICU, and 5.9% needed invasive mechanical ventilation. All patients were treated with antibiotics, 88.2% with antivirals, and 52.9% with corticosteroids. Asthenia was more frequent among children than adolescents (96.0% vs. 55.6%; p < 0.01). Tuberculosis was observed in none of the children, but 33.3% of the adolescents (p-value = 0.003). The percentage of hospitalized patients with family members infected with SARS-CoV-2 was smaller among children than adolescents (8.0% vs. 44.0%; p-value = 0.01). Other variables that differed between children and adolescents were C-reactive protein, creatinine, and need for non-invasive mechanical ventilation. There were no deaths among participants. Conclusion: in our hospital, COVID-19 was most often not severe in children and adolescents. Overall, children were hospitalized more frequently by COVID-19 than adolescents, with some differences for clinical characteristics.