You state both children live with mom, and dad only has them on weekends. You are only doing the moms return, so don't worry how the dad files. your responsibilities are to file the moms return correctly. If your clients return was rejected, because someone claimed them on their return then you must paper file a correct return claiming both children (unless she wants to let the dad claim one with a 8332 form) She will receive all credits entitled to, and file as HOH if qualified. In your scenario the Tie Breaker rules have nothing to do with it. Also what dad does has nothing to do with anything, and she will be rejected again if she put a dependent on her return that is on another return already efiled. Just do your clients return, and it will have to be mailed in, and the IRS will sort it out (will probably take months).