You cannot get any of the second stimulus check since it was based on a 2020 return and the third stimulus payment will be received on the 2021 return for any child born in 2021 .... you cannot amend the 2020 return for the stimulus payments.
As for the advance of the 2021 CTC eventually you should be able to use the IRS portal to add the child however if you cannot then you will simply get it on the 2021 return ...
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
