How do I get my children back in custody violation.

Your facts are a bit vague, but the usual answer if someone is in violation of a decree is to file a motion in family court; do not be bullied.  If he refuses to comply with the existing order, he could lose even joint custody.  But it wold almost certainly be wise for you to consult with a family law specialist before taking any action, so your specific paperwork and circumstances can be reviewed, and all of your options can be made clear to you.