Should we take the kids to see their mother in prison?

In september of last year my fiance's ex-wife was sent to jail and has since be sentenced to 9 years. They have 3 children together which I was well aquainted with them before this time. The kids have been living with us and my fiance received sole custody from the court. His ex is quite the manipulator and liar. This has been apparant for some time with things the kids used to tell us. She was on the run from the law before her final arrest though she lied to my fiance about it and moved the kids several times and even had them living in a motel before her final arrest. Of course she began to kiss my fiance's when she was in jail writing the typical jail letters. She writes the kids every week and at first we withheld many of the letters as she spoke of getting out soon and hoping to be home by Thanksgiving, etc. We knew the reality of the situation thanks to public information and the internet. We have given the kids other letters and have explained to them the situation and they are free to write her whenever they like. We have not allowed any phone calls as they are quite expensive but have been considering if that would be a good thing. Given her track record and her want and need to get in their heads and cause problems we are worried how that would affect the children. Her last letter had mentioned wanting my fiance to bring the kids to visit her when she gets moved to prison as she is still being held in a local jail. We are worried about letting the kids visit her because of her antics in the past and what all she will try and fill their heads with. They seem to be doing so well and have adjusted and enjoy having structure and security. Any suggestions would be greatly appreciated.