There is no valid reason that I know of and there has not been any significant change in circumstances.
Can she make changes like this to the decree without my consent?
I am unable to give you legal advice on divorce. I can give general divorce help for men, though, my knowledge is based on Indiana child custody laws where I am licensed to practice.
Courts typically like to see cooperation between the parties so if the change is requested in order to allow the children to attend an extracurricular activity or if your former spouse's work schedule has changed, for example, the court may not look favorably on one party withholding consent to such a change.
Divorce Resources:State Child Custody Laws
If there is no such reason provided for the change, if your former spouse withholds parenting time, or attempts to make a unilateral change to the decree, then you may be able to pursue an action for contempt.
In order to modify the specific terms of the decree, the parties must agree in writing to the modification or one party must file a Motion to Modify Parenting Time with the court.
Remember, I am unable to provide you with anything more than divorce tips for men, so please consult with a divorce lawyer in your jurisdiction.