Hello Tanya,
In order to claim the children for 2020, you should file an amended tax return by mail and claim the children.
When you submit the tax return, you should include a covering letter that explains why the children should be claimed on your tax return. The IRS will then adjudicate your case and determine who has the right to claim the children.
You may find the links below useful in formulating your arguments to the IRS on why your ex-spouse should not be allowed to claim the children and in guiding you through the process.
https://www.eitc.irs.gov/tax-preparer-toolkit/frequently-asked-questions/divorced-and-separated-pare...
https://www.irs.gov/identity-theft-fraud-scams/identity-theft-dependents
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