To add to @VolvoGirl's reply, keep checking the IRS website below. The IRS is working on a portal that will allow you to update information such as a child being born in 2021 which could make you available for an advance payment in 2020 of your 2021 Child tax credit.
https://www.irs.gov/credits-deductions/child-tax-credit-update-portal
Hope this helps.
