Can I stop my ex from taking my kids out of state?

In Missouri, absent any exigent circumstances, a parent seeking to relocate a child's residence for more than 90 days must give any other party with custody or visitation rights written notice, by certified mail, of the intent to relocate the child. The notice has specific requirements, including the address and phone number of the new residence, the date of intended move, the reason for the relocation, and a proposed revised custody or visitation schedule. The notice must be given at least 60 days before the relocation. Any other party having custody or visitation rights may object to the move by filing a motion with the court that originally entered the custody or visitation order within 30 days of receiving the notice. Failure by the relocating parent to give proper notice will be considered by the court as a factor in deciding whether custody should be modified and whether the child should be returned.