You split in March of 2021? If all of you lived together for all of tax year 2020, it is not clear whether your "ex" could claim both children. He could only claim your daughter (the one who is not his child) if she lived with him the whole year in 2020. Was that the case for 2020? Did you file a 2020 tax return for yourself?
You can try to change which children you are claiming on the IRS Manage Payments portal for the child tax credit if you are trying to get the monthly payments for 2021. Your ex should opt out of the payments so that he does not have to pay them back next year. Amending your 2020 return will not accomplish that since it takes at least six months for the IRS to process an amended return. As for whether you can/should amend to claim both children for 2020---that is a different issue and we do not have any facts regarding that issue.
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-t...
If you do not receive the monthly payments you will be able to get the CTC on your 2021 tax return.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
