Verbal Contract Broken

My fiance and I are getting married in TX in October. We live in MN, and his ex-wife and kids live in IA. He shares joint custody with her, and the weekend of our wedding is our weekend to have the kids. She verbally agreed to let us take them to TX with us so they can be part of our wedding ceremony. We went ahead and bought plane tickets. She has since changed her mind and is refusing to let us take the kids or have them be in our ceremony. Can I sue her for reimbursement for the plane tickets since I never would have purchased them without her verbal agreement to let them come with us? If I can, what are the steps I need to take in order to do so.