If I understand correctly you claimed the children but he was supposed to. I am going to assume that the children live with you. In that case you have to amend your return to list your children on your return but check the box saying that you are not claiming them. That way you can still file as Head of Household and claim EIC and child care credit if otherwise qualified. You give your ex a signed form 8332 giving him the right to claim the children and receive child tax credit. He will have to mail his return with an explanation that you are amending yours to allow him to claim the dependents. Amended returns take 3-4 months to process.
