My DD split with her husband and came back to England with her two children, leaving him in Australia. He didn’t want to come back. The children were 8 and 11 at the time, and having their whole lives turned upside-down was very confusing for them.
However, my DD was careful never to criticise their father,and made sure that they had regular internet chats with him. He came to visit as often as possible.
There was a big hiccup when he found a new GF and brought her over, and the children were upset when he said he was getting married. It’s amazing how resilient children are, though. They grew to accept the situation and have a good relationship with everyone now.
Your GD, with luck, and a big effort from both her parents, will come to terms with the situation. Children are very accepting if they know that they are loved.