Cognitive theories emphasise the role of dysfunctional beliefs about sleep in the development and maintenance of sleep-related problems (SRPs). The present research examines how parents' dysfunctional beliefs about children's sleep and child dysfunctional beliefs about sleep are related to each other and to children's subjective and objective sleep. Participants were 45 children aged 11-12 years and their parents. Self-report measures of dysfunctional beliefs about sleep and child sleep were completed by children, mothers and fathers. Objective measures of child sleep were taken using actigraphy. The results showed that child dysfunctional beliefs about sleep were correlated with father (r = 0.43, p < 0.05) and mother (r = 0.43, p < 0.05) reported child SRPs, and with Sleep Onset Latency (r = 0.34, p < 0.05). Maternal dysfunctional beliefs about child sleep were related to child SRPs as reported by mothers (r = 0.44, p < 0.05), and to child dysfunctional beliefs about sleep (r = 0.37, p < 0.05). Some initial evidence was found for a mediation pathway in which child dyfunctional beliefs mediate the relationship between parent dysfunctional beliefs and child sleep. The results support the cognitive model of SRPs and contribute to the literature by providing the first evidence of familial aggregation of dysfunctional beliefs about sleep.