Kids' Access to Primary Care Act of 2021 This bill modifies payments for Medicaid primary care services. Specifically, the bill applies a Medicare payment rate floor to Medicaid primary care services that are provided after the date of enactment of the bill and extends the payment rate to additional types of practitioners (e.g., obstetricians). The Centers for Medicare & Medicaid Services must conduct a study on the number of children enrolled in Medicaid, the number of providers receiving payment for primary care services, and associated payment rates before and after the bill's implementation.