A lot depends on where the original orders were done. It sounds like maybe Kansas? If so, a modification would have to be done in Kansas, or in Nebraska once the kids have been there 6 months. Depending on what the original custody orders say, you may be able to enforce his visitation rights, but he would have to talk with an attorney in Nebraska for that.
Sign up to receive a 5-part series of useful information and advice about child custody law.