Go back to your divorce attorney or some other attorney that practices family law and act now. File in the same Court that granted your divorce. Discuss custody modification, contempt, how the UCCJEA will play into your case and emergency relief. Don't wait another day. Go now. There are too many facts, issues and questions for a free, on-line Q&A.
I agree with Mr. Thompson. Every single day that passes that you don't lawyer up and take immediate action may result in Mississippi no longer having jurisdiction over the children. What's worse is that California may obtain jurisdiction. Where would you rather fight this battle - in Mississippi or in California? Do not delay any longer! Get representation NOW!
No attorney-client relationship has been formed by this answer. The answer given herein should not be your only inquiry into the matter or issue. You should schedule an appointment and seek the advice of an attorney to fully explain the facts so that the attorney can make an informed analysis and recommendation concerning your problem and advise you of all of your rights.
I agree with the other guys. You really need to file for contempt and modification. The longer you wait, the more likely the chancellor will not change custody as the children get more stable in the community where they are currently. With both of you having moved from Mississippi, you also potentially have an issue of another court possibly taking jurisdiction of the matter.