I haven't read through all the post, as yet, but to respond to the original question, I think it's basically up to the couple. If they agree that the kids are to come first, great. But I they decide that they're going to put their marriage first, that's fine, too. In my view, he most important thing is that they're on the same page about this and it's not a source of conflict
That being said, I don't believe that prioritizing one factor should mean that anyone gets hurt. I agree with you, OP, that it can be hard for under 5s to be away from their parents for an extended time period. And so, I don't think this is fair to either the child or you, the person who will be taking care of her/dealing with her feelings.
But what can you do? Nothing but love her and entertain her as much as possible during the visit. Please let us know how it goes..