divorced parents and claiming chil

my husband and i have been divorced for about 7 years,, in our divorce papers it states that he is to claim one dependant child and i am to claim one,,, at the time they were living with me,,, we have joint custody. now they live with him,,, but the only legal document states that we share them on taxes,,,, the last 2 years he has claimed them both without my permission,,, he isnt supposed to do this,,, is there anything i can do about this and will he have to pay me back if i take him to court,,, thank you

what are my options???