The "add a child" function does not seem to be ready yet.
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
If you are not able to update your status, you would get the credit when you file a tax return at the end of the year. Your child must have a social security number, so if you don't have that yet, you need to apply as soon as you can.
