I'll try to answer your question by describing reunification therapy.
If we're talking about an intensive form of therapy, not in a psychologist's office but in what people are calling camps, even though they're not really camps, a typical scenario would be that the child is not told that they're going to go anywhere for treatment, but they are picked up by youth transport service workers, perhaps as they come out of school or perhaps at one of the parents' houses.
When they ask what's going on, they're told they'll find out when they get there. They're told that if they don't co-operate, the workers have handcuffs. The workers can chase them down and take them down. Their phones, IDs and money are taken away, and they are taken by plane or some other way to an unfamiliar city, where they will go to a hotel or an Airbnb, not a regular clinic or hospital setting.
Once there, the parent they're avoiding also shows up, and they will spend four days being told that they cannot talk about anything that ever happened before. They can't explain why they don't want to be in contact with the parent. Instead, they have to watch videos that will tell them that they have been totally confused and over-persuaded by the other parent and that they can't trust their own feelings about this.
After several days of this, they are told they are going home with the parent they don't want to be with and they may have no contact with the parent they prefer. If they have any contact with that person, he or she, but mainly she, can go to prison for being in contempt of court. That's the treatment.