Before the implementation of the Affordable Care Act (ACA), most children in low-income families were already eligible for public insurance through Medicaid or the Children’s Health Insurance Program. Increased coverage observed for these children since the ACA’s implementation suggest that the legislation potentially had important spillover or “welcome mat” effects on the number of eligible children enrolled. This study used data from the 2013–15 American Community Survey to provide the first national-level (analytical) estimates of welcome-mat effects on children’s coverage post ACA. We estimated that 710,000 low-income children gained coverage through these effects. The study was also the first to show a link between parents’ eligibility for Medicaid and welcome-mat effects for their children under the ACA. Welcome-mat effects were largest among children whose parents gained Medicaid eligibility under the ACA expansion to adults. Public coverage for these children increased by 5.7 percentage points—more than double the 2.7-percentage-point increase observed among children whose parents were ineligible for Medicaid both pre and post ACA. Finally, we estimated that if all states had adopted the Medicaid expansion, an additional 200,000 low-income children would have gained coverage.