Someone is filing my children without my knowledge

So you fie claiming them, You will need to mail it in with documentation showing that you have the right to claim them.

If you think that you are the one who can claim the dependent, then file your tax return and claim the dependent. IRS will then contact both of your and figure out who must amend the tax return and pay the tax due with interest and penalty.

Child of divorced, separated or unmarried parents: Only the custodial parent can claim the child. It does not count who is paying child support. The custodial parent is the parent with whom the child lived for the longer period based on the nights spent. The other parent is the non-custodial parent. Non-custodial parent can claim the child only if the custodial parent signs a release Form 8332. With form 8332, the noncustodial parent gets exemption deduction and child tax credit, but not the head of household status. Even after signing Form 8332, the custodial parent is head of household and get EIC and any child care credit. Your U.S. Tax Return: Child of separated or divorced parents (http://taxipay.blogspot.com/2008/03/child-of-separated-or-divorced-parents.html)

File A return and mail it to IRS and they will investigate you and the other tax payer to see who must claim them. They will only investigate if you File.

This has become a common question with electronic filing. When you submit your return electronically (which is being required this year), the IRS checks the SSNs of all dependents to see if there is a previous filing. If there is, the electronic return is rejected. You will have to mail the return in such a case and have the IRS resolve the situation.