You are not due a child tax credit for the child you claimed in 2020. Since you received an advance payment on the child tax credit for that individual, it is applied to the child tax credit you are due in 2021 for the other child. While it is true that you would not have had to pay back the advance for the 2020 dependent if you had no child tax credit in 2021, it is applied to your 2021 credit since you have one. Another way to look at it is the IRS does not keep tract of who the child tax credit belongs to, rather you calculate the credit you are due for 2021, and any payments you received towards your child tax credit is used to satisfy that obligation by the government.