Keep in mind, an accepted tax return is not a guarantee to also have the right to claim the dependents on that return. If your ex erroneously claimed your child as a dependent, the Internal Revenue Code allows them to amend their tax return within three years of filing the original or within two years of paying the relevant tax, whichever is later. However, eliminating the dependent generally increases their taxable income and may require them to pay additional tax for that year. Although penalties may apply to the underpayment, the IRS can waive them if they can convince them that it was an unintentional error.
If they choose not to amend their tax return, they run the risk of the IRS discovering that the same child is being claimed as a dependent on two returns. The IRS has three years from the time they file the original return to perform an examination and make additional assessments. In the event they are chosen for an audit, the agency is likely to require proof that your child either lives with them or that they have your consent. There is the possibility that the IRS will not discover the error within the three-year period. However, if they claim a dependent with full knowledge that they do not qualify, the IRS may argue that it has an unlimited amount of time to examine their return since they made a willful attempt to evade income tax.