Background: Offspring of parents with severe mental illness (SMI: schizophrenia, bipolar disorder or major depressive disorder) have an increased risk of developing mental disorder themselves. In childhood they may have neurodevelopmental delays, cognitive deficits and social adversities. We aimed to investigate if these individuals are more at risk of being diagnosed with a mental disorder during childhood/adolescence in a national sample. Methods: By linking Danish registers we established a cohort consisting of all persons born to parents with SMI with those born to parents without SMI serving as a reference group. Incidence rate ratios (IRRs) for offspring diagnosed with a mental disorder by parental mental disorder were calculated. Results: Offspring of parents with SMI showed increased IRR for all diagnoses of child and adolescent mental disorders compared to the reference group. Offspring of mothers with schizophrenia had IRR of 2.60 (CI: 2.50-2.70, N = 2550) of having any diagnoses, for children of fathers with schizophrenia IRR was 2.06 (CI: 1.97-2.16, N = 1901) and for offspring of two parents with schizophrenia IRR was 4.57 (CI: 3.94-5.31, N = 175). For individuals with a mother with bipolar disorder the IRR was 2.29 (CI: 2.09-2.50, N = 502), with a father 1.77 (CI: 1.74-1.87, N = 320), whereas the IRR was 2.96 (CI: 2.63-3.34, N = 264) if both parents had unipolar depression. Discussion: Offspring of parents with a SMI have a higher risk of being diagnosed with any child and adolescent mental disorder. The IRRs for all diagnoses during childhood were increased by a factor 2-4. Having two ill parents increased the IRR.