There is only one child credit, it is calculated on your 2021 tax return based on your 2021 income and dependents you claim in 2021.
The IRS was directed to make advance payments if possible, and to use whatever information was available to them, such as past tax returns. But neither the advance payments, nor the credit itself, is tied to specific individuals. It's all just dollars.
If you qualify for a $6000 credit, and you received $4800 in advance payments, you should get another $1200 added to your refund. You don't get to say that part of the $4800 was for a child you aren't claiming so that should be covered by repayment protection, it doesn't work that way and was never meant to. If based on your tax return you are entitled to 6000 dollars and you got 4800 dollar already, you get 1200 more dollars. It's just money, it has no ties to specific dependent.
