I think I may have answered your question in another post. The short answer is the father needs to file a Petition for Modification of Custody in the Superior Court of the Mother's residence, and get the Mother served with the papers as soon as possible. You will need to show the Court that there has been a substantial change in circumstances since the entry of the Order granting the Mother custody of the children.
The Court will also likely appoint a Guardian Ad Litem to look after the best interests of the children. I can advise you on the steps and costs involved if you would like to discuss things further in a private and confidential consultation at no cost to you. Please do not hesitate to contact my office.
Please know that this is a general interest posting and does not create an attorney-client relationship.