Focus ONLY on your situation and stop worrying about the EX ... that is their problem.
As for the first 2 stimulus payments ... if you did not get them in advance and you claimed the kids on the 2020 return the credit would be on the 2020 return line 30. If you did get them and you don't claim the kids on the 2020 returns then you get to keep the money ... no harm no foul.
As for the third payment ... again if you did not get it and you claim the kids on the 2021 return the credit will be on the 2021 return. If you did get it and you will not claim the kids on the 2021 return you get to keep it ... again no harm no foul.
Now for the ADVANCE payments ... if you claim the kids on the 2021 return then the credit will be reconciled and any missing payments will be part of the 2021 credit. If the ex gets the advance payments and will not claim the kids on the 2021 return then they may need to payback some or all of the credit they are not entitled to ... again this is not your problem.
Read all the rules here : https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-f...
