A child cannot make such a decision. Both parents can agree and if so the agreement should be reflected in a modification of the existing judgment. If the parents don't agree, this can only happen through a custody modification. The child can get a voice through a custody evaluation, through an attorney appointed for the child or (as a last resort) by testifying to the court. In any case, please see an experienced family law attorney as soon as possible to get further guidance.
Kramer & Associates