Oh how sad! I can imagine how that must feel. Mind you; I am single (not that I want to be) and have no children (but I really wanted to). So I can only imagine it and it's sad.
This feels like abandonment, you write. But you also write you had a very good relationship.
You write down the reasons for them to move, but not how they expressed their feelings. Did they show you how they feel about it?
They must have been thinking about this a long time and it would have been nice if they had discussed this with you sooner. But this didn't happen and you are now confronted with their decision, which may make you feel mostly the bad things.
Were you able to discuss your feelings or were you too upset?
It is natural to feel upset. It would be good if they let you cry, lament, and whatever you feel like doing after this news. This is a time to be sad; it's a natural reaction.
Maybe if they share more about their move, show you photos, show you how you can engage, help you set up a laptop for instance so you can do video meetings, this would help. It won't feel the same, it won't be the same. But if they help you coming to grips with this situation it will help you heal.