Child Tax Credit Improvement Act This bill amends the Internal Revenue Code, with respect to the child tax credit, to: (1) allow taxpayers an increased $3,600 tax credit for each young child under the age of six (young child tax credit), subject to specified limitations based on adjusted gross income; (2) require the Department of the Treasury to establish a program to make advance payments of the young child tax credit; (3) modify the refundable portion of the child tax credit; and (4) require annual inflation adjustments for both the child tax credit and the young child tax credit.