First, if he has already e-filed, claiming those kids, then you will not be able to e-file, because the IRS e-file system tracks the use of every Social Security number and does not allow them to be used on two different returns.
So you will have to print your return and mail it. Eventually, the IRS will noticed that the kids have been claimed by two different people and send out letters to determine who has the right to claim them.
You might look at the Taxpayer Advocate Service, although under the circumstances, they may not be able to tell you anything different from what I have said.
Finally, since you are presumably missing the Child Tax credit and/or the Earned Income credit (at least until the IRS straightens this out months from now), you might contact an attorney to see if you have a claim for relief in a civil court. We do not give out legal advice here in the Community.
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