For tax purposes, a child that is born during the year is considered to have lived with you for the entire year, so long as he lived with you from birth through the end of the year. You actually should amend your 2020 return to get the additional credit to which you are entitled.
The bad news is, that's not going to happen in time to fix anything that is preventing you from receiving the monthly advance payments. But you will definitely get the credit on your 2021 return.
Nick K, EA
