First and foremost - you should be working directly with your ex and not his wife....this is not said as an attack on your methods, simply as a way to direct your efforts a little better.....the children are you and your ex's responsibility and the two of you are the parties named in the custody/visitation agreements - not her. As a stepmother she has no place in the politics of your efforts to communicate with their father - I know that will take some heat from some step-parent's out there, but in this case you are getting nowhere by dealing with her and so need to be more direct in the communications.
Legally there may be little you can do - other than talking with an attorney to see if a LONG EXPENSIVE and probably completely USELESS court battle would result in anything for you. Your children's expressed reluctance to visit could be considered if you did go to court (to request modification of the visitation order) - a judge may consider their opinion (depending on their ages)and it is possible that even if no change were made the judge may opt to give a little "lecture" to the father and leave it to him to pass the information along to the parties involved in the badmouthing and leave it to him to correct the situation.
As I said, any court proceedings would likely be long, expensive and highly ineffective, but if you feel committed enough to this you will want to talk to a lawyer in your area -- most offer free initial consultations and at least you could get the opinion of someone with experience in your jurisdiction with the judges opinions on these matters.
Good luck and God Bless
