Based on your brief description of the circumstances, it would be wise for you to consult an attorney to help you. Under the Uniform Child Custody Jurisdiction and Enforcement Act, if your original custody orders issued out of a Missouri court, then the Missouri court retains continuing jurisdiction over custody issues. There are some exceptions, but it is unclear based on the information you have provided whether the exceptions applyh in your case.
In any event, the children's mother, irrespective of whether the custody decree is enforced in Missouri or in Kansas, would, in most cases, be required to notify you well in advance of her intended move out of state. Once she does, you can file an objection to relocation in the court that has jurisdiction over ongoing custody matters. The court will examine a host of factors to determine whether the custodial parent should be allowed to relocate out of state with the children, or whether a modification of the original custody orders is warranted.
Again, you would likely want to consult an attorney regarding your specific circumstances. If you have already received notice of the custodial parent's intent to relocate, you will want to consult an attorney right away, because in many cases there is a time limit for you to file your objection to the relocation; if you do not file within a certain amount of time, then you may have waived your objections to the move.
DISCLAIMER: The foregoing information is not intended to create an attorney-client relationship, nor is it intended as advice to your or any reader's specific set of circumstances. Rather, it is intended to provide information regarding a general set of circumstances. You or anyone reading the foregoing answer and information should consult with an attorney regarding your specific situation.