I am not understanding what you are going for unless it is shear vindictiveness which is not a legal basis for a fraud claim. If you made insufficient income to claim your two children on your income tax filing you lost the right - or had no longer had a legal right - to claim them, correct? As you lost the right to claim the two children on your income tax filing, did not that reinvest your ex-spouse, who did make income sufficient to file an income tax filing, with the eligibility to claim his children on the tax filing? He did not violate your right to claim them - as you may have done had you filed - as you did not have the capability to file a tax return.
Unless there is more to your story than mentioned in your post, you have no claim of any kind.
The rules regarding this subject are complex. In general, a noncustodial parent may not claim a child as a dependent unless the custodial parent relinquishes, in writing, the right to take the deduction. You have no obligation to report your knowledge that the taxpayer claimed an extra deduction, but if you chose to do so, instructions on contacting the IRS are available online. It's hard to see why you would choose to do so, but that's your business.