Ex wife revoking visitation due to being stationed in Germany for 3 years?

My husband found out in January that he was going to be stationed in Germany. Our first worries were that his 2 young sons (5 & 6 years old) were going to be robbed of their time with daddy. We informed his ex wife (whom he has joint custody with) of our next duty station being in Germany. At first she was okay with us taking them Germany for the summer, considering they both agreed to the father having them for that duration. We do not have proof of her being okay with this trip at one point, and last week she blatantly informed my husband during his phone call with his boys that he was not going to take them to Germany. This of course upset the kids. He went active duty March 17th, 2010, while they were still married. They did not file for divorce til that following May. (he was in the National Guard prior to going active duty, and was deployed once before (not to be confused with stationed considering this was an non-accompanying tour in 2002 when they first got married) It is common fact that a member of the military could be stationed overseas. When they did file for divorce last year, he was already stationed out of state going through training, so she knew that our chances of being stationed back in KY (where the children and their mother reside) was a slim one. She has already mentioned once in a previous email before we even found out that we were going to Germany that she was having second thoughts on us getting the kids for the summer. She threatened that she would file a motion that would restrict his summer visitation rights that were already in order. Due to the fact that we are going to have to pay for the father plane ticket, along with the kids plane ticket, we can not afford attorney fees to address this issue before it is too late. We can not take the children out of the country without notarized permission from the mother. So now that she has denied us to take the kids overseas, we have no choice but to pursue this in court. I wanna know if we have a case against her to get it to where my husband can have his kids, or are we going to waste or money and get denied by the courts as well. The custody agreement has no specific details on anything pertaining to military, or duty stations. It merely states that he will have the kids 1 week after school is out in the summer til the 2 weeks before school is back in session. We need advice ASAP!!!! Please help us in the right direction, and give us some pointers on what we should do, or maybe how we should approach her so that we can resolve this as adults, without having to waste anymore time or money in the courts.