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.

Our family had a similar issue, with our child born in December of 2020.  We marked “1month” as the time the child had been living with us in 2020, and so only received the $500 tax credit, and did not receive the $300 monthly credit in July.   However, we have recently been informed that we should have selected “12 months” for any child born in 2020.  So we submitted an amendment to our 2020 taxes, making this change, and now will be receiving an additional refund.  

Hopefully the above helps for your situation,  but I also have an additional question.  Since we amended our 2020 tax return to include our dependent child born in 2020, will we automatically start receiving the $300 monthly credit going forward, and for the credit that we should have received in July?  Or do we need to take additional action with the IRS to communicate this update to receive the monthly credit?  Or,  will the monthly payments instead be given in a lump sum refund when we file our 2021 taxes?

thanks for any help.  

Thank you Nick. I’m trying to amend using the app but having difficulty. How do I actually change from the ‘other dependent tax credit’ to the child tax credit and put how long my child lived with me? (12 months as you said) I’m not even worried about getting the advance payments, just want to be sure I’m set when I file for 2021.

thanks again !