Background: Knowledge of COVID-19 and the pandemic's effects on Danish children's body weight is limited. Objective: Objectives were to investigate (I) risk of weight changes among Danish children with and without SARS-CoV-2, (II) associations between weight changes, psychological symptoms, and long COVID symptoms, and (III) weight distribution pre- and post-pandemic. Methods: A national survey was administered to all Danish children aged 0–18 years, with prior COVID-19 (cases) and matched references including questions on weight, weight changes during the pandemic and long COVID-related symptoms. Descriptive statistics and logistic regression were used. Weight distribution was compared with a pre-pandemic database. Results: In all, 17 627 cases and 54 656 references were included. The 4–18-year-old cases had lower odds of unintended weight gain. The 2–3-year-old cases had higher odds and the 15–18-year-old cases lower odds of weight loss compared to references. Regardless of COVID-19 status, any reported long COVID-related symptom was associated with a change in body weight. No sign of increasing obesity rates was found among Danish children post-pandemic. Conclusion: COVID-19 was associated with higher odds of weight loss in 2–3-year-olds and lower odds of unintended weight gain in 4–18-year-olds. Any long COVID-related symptom was associated with higher odds of weight changes regardless of COVID-19 status.