Responses
I'd agree if she were closer to 10 but she's 5 and still hasn't made any progress with her abandonment issues (caused by dad) since she started acting up when she became attached to Kaisei. And the dad most likely doesn't have a residence in Japan since he lives in America. Taking a still traumatised 5 year old and moving her to not only a new household but a new country with a diff language would not be wise
I think yui's mature enough to know what you guys are talking about. Maybe like give him a week or a month of taking care of yui, And if yui still didn't like being with him for a week or a month then it's safe to say that she's staying with y'all. You can't force her if she doesn't want to, that'll just form an unnecessary conflict.