Background: The incidence of pediatric neuromyelitis optica spectrum disorder (NMOSD) and myelin oligodendrocyte glycoprotein (MOG) antibody-associated disease have not been reported previously. Our aim was to estimate the incidence of pediatric NMOSD and the occurrence of anti-MOG antibody-associated disease in Denmark during 2008–18, and to evaluate the diagnostic usefulness of antibodies against MOG and aquaporin-4 (AQP4) in children <18 years. Methods: We undertook a nationwide, population-based, multicenter cohort study using data from the Danish National Patient Register, the Danish Multiple Sclerosis Registry, and laboratories providing anti-AQP4 and anti-MOG antibody analyses. Diagnoses were confirmed by review of the medical records, including blinded MRI review in most children with acute disseminated encephalomyelitis (ADEM). Results: In children with acquired demyelinating syndromes, anti-AQP4 antibodies were detected in 4% and anti-MOG antibodies in 18%, including in the two children with ADEM who relapsed. We identified four children with NMOSD, equivalent to an incidence of 0.031/100,000 (95% confidence interval = 0.011‒0.082). In anti-MOG antibody-positive children, 32% relapsed during follow-up. Conclusions: Pediatric NMOSD and MOG antibody-associated disease are rare, but one-third of anti-MOG-positive children relapsed. In pediatric ADEM, only anti-MOG antibody-positive children relapsed, but the overall risk of relapse after pediatric ADEM was low.