A child who is born this year is eligible for the child tax credit. If you do nothing now, you'll receive the credit when you file your return. A newborn is also eligible for advance child tax credit payments, but the IRS has not yet set up the mechanism to tell them about the child. They expect to have this feature available late in the summer, at which time it will be on the portal:
https://www.irs.gov/credits-deductions/child-tax-credit-update-portal
If the child was born in 2021, then you can't add that child to your 2020 tax return. You will just get any credits due when you file your 2021 tax return, which you can't do until 2021. In other words, you will get any credits due. You just won't get them in advance.
A child who is born this year is eligible for the child tax credit. If you do nothing now, you'll receive the credit when you file your return. A newborn is also eligible for advance child tax credit payments, but the IRS has not yet set up the mechanism to tell them about the child. They expect to have this feature available late in the summer, at which time it will be on the portal:
https://www.irs.gov/credits-deductions/child-tax-credit-update-portal