The other parent has been claiming the children in the past years, this year It was my turn to claim them but it was too late when we agreed, I already filed the taxes. Is it possible to do it?
It depends. If you e-filed your return, it's in "pending" status. You won't be able to change it. It's like putting a letter in the mailbox. You can't retrieve it.
If your return gets rejected, you'll be able to change your tax return.
If your return gets accepted, you'll need to amend your income tax return.
NOTE: At this time, you're not able to amend a 2020 income tax return. As of today, it's estimated that the IRS will finalize the 1040X form on 03/25//2021. However check the forms availability to see any updates to that date.
Generally, Form 1040-X, Amended U.S. Individual Income Tax Return, must be filed within three years of the date the original return was filed, or within two years of the date the tax was paid, whichever is later.