Creates a new refundable monthly child tax credit and a Treasury/IRS-administered program to deliver it as monthly advance payments. The law establishes how much is paid each month, income-based phaseouts, identity and eligibility rules, protections for certain payments, reporting and notice requirements, and special rules that apply in U.S. possessions. The Treasury (through the IRS) must issue implementing regulations and may share limited tax return information to run the program.