A dependent can only be claimed on one tax return. The parents or courts will need to work out or decide who can claim the dependent children each year.
If there is a dispute I would recommend contacting the IRS. The IRS will investigate who really should be getting the deduction. At that point, they will make a decision based on the criteria below, and the taxpayer who incorrectly claimed the dependent will be assessed any additional taxes and penalties, and required to file an amended return. If you would like to call the IRS and report this, their number is 1 (800) 829-1040.
