Public health insurance for low-income children in the United States is primarily available through Medicaid and the Children’s Health Insurance Program (CHIP). Mixed eligibility occurs when there is a mix of either “Medicaid- and CHIP-eligible” children or a mix of “eligible (for public insurance) and ineligible (for public insurance)” children in the family. We used data from the Medical Expenditure Panel Survey (MEPS) Household Component for 2001–12 to examine insurance coverage, access to care, and health care use for eligible children in families with mixed-eligible siblings compared to those in families where all siblings were eligible for one program. We found that mixed eligibility has a significant dampening effect for eligible children in families with a mix of eligible and ineligible siblings. These children were more likely to be uninsured and less likely to have a usual source of care, less likely to have any preventive dental or well-child visits during the year, and less likely to fully adhere to recommended preventive dental and well-child visits than eligible children with all-Medicaid- or all-CHIP-eligible siblings. We found no significant impact for eligible children living in Medicaid-CHIP-mixed families.