The recovery rebate credit is based on your 2021 income. People received those $1400 EIP payments early last year because they were in a hurry to send out those stimulus checks. But they were an advance on a credit you can get on a 2021 return.
You say your ex got the stimulus "for the children"---but you do not mention if you filed a joint return last year or if YOU got the $1400 for yourself. If YOU did not get it then you can get it with your 2021 return.
Better check your IRS account to see what it shows there
https://www.irs.gov/payments/your-online-account