Last year tax return

[Chanta28]

What if someone else claimed my child as well as myself 

[DoninGA]

Did you also file a tax return claiming your children as dependents?  If so, did you receive a letter from the IRS requesting you to amend your tax return if your filed return was incorrect?

Or are you just asking what to do if someone filed a return improperly claiming you and your children?  

You can report a fraudulent tax return using the information from this IRS website - https://www.irs.gov/individuals/how-do-you-report-suspected-tax-fraud-activity

[TomD8]

If the IRS receives two tax returns, each claiming the same dependent, they will contact the taxpayers involved and request documentation.  The IRS will then determine which of the two taxpayers has the rightful claim.  The process may take a few or even several months.

If your e-filed return is rejected because someone else already claimed the same dependent, you should resubmit your return on paper.

The right to claim a child as a dependent belongs to the custodial parent, which to the IRS is the parent with whom the child lived more than half the year.  The IRS definition of "custody" has nothing to do with custody awarded by a local court.