I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a dozen years of practice experience handling matters of this nature. It is a pleasure to assist you today.
You first need to send her a certified letter with return receipt in which you point out the language that she is in violation of relative to the visitation rights. Ask that she honor those rights and tell her the letter should be considered legal notice of your position.
Give her 7 days from the postmark to respond in the affirmative with a reasonable visitation schedule that adheres to the decree. Tell her in the letter if she does not, you will take her to court and seek custody, if need be.
Unfortunately, if she does not oblige the letter, your only option is to file a contempt motion against her for violation of the court order. You would have to serve the contempt motion on her and establish a court date. The court can exercise jurisdiction over her and order her to return with the children.
In short, it is not an easy road, but you are on the right side of the law here. You will ultimately win and you are to be commended for trying.
Let me know if you have any other questions or concerns. Please also rate my answer positively (THREE OR MORE Stars) on the ratings bar on your end so I can receive credit from the site for my response.
Thank you and best wishes!