The child tax credit (CTC) has grown to become a core component of American family tax and welfare policy. Temporary expansions made the credit fully refundable in 2021 – even very low-income families could receive the maximum benefit. On a conceptual level, full refundability transitions the CTC from being a conventional tax credit to something resembling family and child allowances common elsewhere. Implementing a tax credit designed to resemble a child allowance as closely as possible raises potential administrative challenges. The purpose of this report is to explain those challenges and offer principles for efficiently administering a permanent child benefit in the US policy context.