In brief, there is no way to claim the children on your tax return without the cooperation of your ex. Your ex needs to give you a signed form 8332 (dependent release) that you use to file your tax return and you must mail to the IRS after e-filing. The IRS does not honor state court custody cases, they only follow their own federal laws and regulations.
You can complain to the family court. The family court can take various actions including forcing the other parent to sign the release, or modifying the support or custody order, or even holding the other parent in contempt. But you have to seek your remedy in family court.
On the off chance that your ex did not deliberately claim the children, and just needs to be educated on what to do, the previous discussion has all the information, or if you post a specific question here, we can clarify anything you don't understand.
