Background: Given the partially shared genetic liability between schizophrenia and bipolar disorder, we aimed to assess whether 7-year-old children with a familial high risk of schizophrenia or bipolar disorder display specific deficits of sustained attention and interference control compared with each other and with control children. Methods: An observational cohort was identified through Danish registries and consisted of 522 children 7 years of age with no, one, or two parents with a diagnosis of schizophrenia or bipolar disorder. Control subjects were matched based on age, sex, and municipality. Sustained attention and interference control were assessed using Conners’ Continuous Performance Test II and a modified Eriksen flanker task. Assessors were blinded to group membership of participants. The effect of higher genetic loading was not considered in the statistical models owing to low numbers. Results: At 7 years of age, children with a familial high risk of schizophrenia displayed deficits of sustained attention and subtle deficits in interference control compared with control children and children with a familial high risk of bipolar disorder. Children with a familial high risk of bipolar disorder displayed similar abilities of sustained attention and interference control as control children except in terms of a lower accuracy. Conclusions: Our findings suggest distinct neurodevelopmental characteristics in middle childhood of sustained attention and interference control for children of parents with schizophrenia or bipolar disorder.