OBJECTIVES: A growing body of research examines how adult children's education influences older parents' cognitive health. Whereas prior studies tend to focus on educational advantage, this study seeks to understand how various measures of educational disadvantage are associated with parents' dementia likelihood. In addition, we ask how the risks associated with one child's educational disadvantage are shaped by a sibling's educational success. METHODS: Using data from the U.S. Health and Retirement Study (2000-2018) and event history analyses, comparisons are made between measures of offspring educational disadvantage and their relationships with parents' risk of dementia onset. In addition, analyses are conducted to understand whether the link between one child's educational disadvantage and parental dementia onset is influenced by a sibling's educational advantage. RESULTS: Educational disadvantage is associated with an increased risk of parental dementia onset, with a threshold measure for whether a parent had at least one child without a high school education providing the best model fit for the data. Moreover, the heightened risks associated with one child's educational disadvantage are not offset by another sibling's educational success. DISCUSSION: Children's educational deficits are a hidden source of health disparities among older parents. Although scholars in recent years have rightly focused on the importance of offspring education, more attention should be paid to conceptualizing how educational disadvantage matters for parents and how the educational attainment of each child shapes parents' cognitive health.