I am away from my notes at the moment, but I believe that Prince Christopher's first wife, who became HRH Princess Anastasia of Greece, made an agreement for her daughter-in-law, HSH Princess Xenia Georgievna's children, to be heirs to the throne of Greece. This is interesting for several reasons. First, Princess Anastasia was a wealthy American widow and as such would not have normally been entitled to be a princess in her own right. She would normally have been Princess Christopher of Greece. However, she joined the family when it was in considerable turmoil, and she was by all accounts incredibly generous to her new in laws. In the midst of this, her son by her first marriage, Billy Leeds, married Princess Xenia. Second, this is the only case I know of where the children of an American man, in this case, Billy Leeds, became heir to a foreign kingdom. As I understand it, Nancy Leeds Wynkoop was in line as is her daughter Alexandra Wynkoop.
By the way, Prince Christopher had only one legitimate son, Prince Michael. I believe Prince Michael is still a Greek dynast but his daughters were the product of a morganatic marriage. Thus, they and their descendants - which means Prince Christopher's descendants - are not in line for the throne of Greece.