Congratulations on the birth of your twins. The IRS is basing the Advanced Child Tax Credit payments on children claimed on the 2020 return or born in 2020. The IRS plans to update the Manage Payments portal to allow taxpayers to update the information on children born in 2021 in late August or September.
You would use the Manage Payments portal to:
Here is the link to the Child Tax Credit Update Portal: https://www.irs.gov/credits-deductions/child-tax-credit-update-portal
Here is a link to the 2021 Child Tax Credit and Advance Child Tax Credit Payments Frequently Asked Questions: https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-frequently-asked-questions
Congratulations on your new twins! Later in the summer there will be a portal where you can enter information for babies born in 2021. The portal is not ready yet. You will need their Social Security numbers. Since you will not begin receiving those monthly payments in July you will be able to reconcile the full amount of the child tax credit when you file your 2021 tax return next year.
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021