Congress authorized expansion of the child tax credit from $2,000 per child dependent to $3,000 for children aged 6-17, and for those children aged 5 and under, $3,600. While plans and methods aren’t finalized, many people will be receiving a portion of that in advance starting in July. The actual process isn’t finalized yet, but it is apparent that in order to receive a monthly payment for your children, you will not have to do anything, as it will be based on your last tax return filed. If you don’t want the monthly, you will need to opt out.
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021