Yes I see your difficulty. She should visit to see the kids of course; it would be better if it were her idea.
She should make the effort here. The kids should not have to travel to her. I was only thinking a neutral location would be better if you had to suggest it. Then she would see the family again without the possible "having to crawl back" feeling. But that is not in the cards.
So I guess letting her know the kids miss her and asking what HER plans are to come visit may be the thing to do. She may have an alternate plan.
Can she afford to come?
Best this all come from you - not the kids. Best not to put them in the middle here.
