Good news!  The IRS has indicated that it will be updating its Child Tax Credit Update Portal to allow you to make changes to your dependent information.  This means you will be able to let the IRS know about your new bundle of joy and start receiving the advanced payments.  The bad news is that this planned update is not scheduled to happen until "late summer".  You will miss the first few advance payments.  Congratulations on your new baby!

Hi,

Thank you for asking your question! You have the option to wait until next year to get your full credit on your tax return. If you would like to receive payments monthly in 2021, you must log in to the IRS portal to do so. Some of the functionality, what you can and can't do, of the IRS portal is being introduced in phases.

Here is where you can get the latest information:

IRS Advanced Child Tax Credit Portal

IRS Advanced Child Tax Credit FAQs