Although all state Medicaid programs cover children's dental care, Medicaid-eligible children are more likely to experience tooth decay than children in higher-income families. Using data from the 1999-2016 National Health and Nutrition Examination Survey and the 2003, 2007, and 2011-12 waves of the National Survey of Children's Health, we examined the association between Medicaid adult dental coverage (an optional benefit) and children's oral health. Adult dental coverage was associated with a statistically significant 5-percentage-point reduction in the prevalence of untreated caries among children after Medicaid-enrolled adults had access to coverage for at least one year. These policies were also associated with a reduction in parent-reported fair or poor child oral health with a two-year lag between the onset of the policy and the effect. Effects were concentrated among children younger than age twelve. We estimated declines in poor oral health among all racial and ethnic subgroups, although there was some evidence that non-Hispanic Black children experienced larger and more persistent effects than non-Hispanic White children. Future assessments of the costs and benefits of offering adult dental coverage may consider potential effects on the children of adult Medicaid enrollees.