The IRS is not likely to respond in this situation. If you are an ex-spouse the service will give the dependency deduction to the first parent to claim it whether they are entitled to or not out of administrative convenience. Your only recourse will be in civil court.
The question is a little confusing. Were these dependents you were entitled to claim on your return? If they are dependents you were entitled to claim you will need to file a paper return with the dependents listed on the return. The IRS will flag the return as the social security numbers were used on a previously filed return. The IRS will then investigate as to who can rightfully claim the dependents.
Circular 230 Disclosure: To ensure compliance with requirements imposed by the IRS, any tax advice contained in this communication, including attachments thereto, was not written to be used and cannot be used for the purpose of (a) avoiding tax-related penalties under the Internal Revenue Code or (b) promoting, marketing or recommending to another party any tax-related matters addressed herein. If you would like a written opinion upon which you can rely for the purpose of avoiding penalties, please contact Steven F. Schroeder using the contact information set forth above.
The IRS is unlikely to respond to a report that someone else has committed tax fraud. If you wish to try to collect an award for being a whistleblower, bo to http://www.irs.gov/uac/Whistleblower-Informant-Award. If you are concerned because these are your dependents, then one of the other fine gentlemen who has already responded has given you excellent advice.