Sometimes kids/teens are unclear about why they need to see a therapist, or they flat out disagree with parent concerns.
We encourage parents to be open with their children about their concerns and not surprise them news of the appointment at the last minute.
It is OK to tell kids that there are patterns that do not seem healthy and your goal as parents is to move towards health (physical, relational, emotional, etc.). When a parent is concerned about their child's health, they contact a professional to find out if there is a problem that should be addressed, and what is recommended to do so. So going to therapy for the first time is a chance to talk about concerns and see what help is needed.
It can be a chance for kids to share their opinions if they are different from the parent, and to get a professional perspective.
You might say to your child, "You are going to have a chance to tell the therapist where you disagree with me, and talk about what you think the problem might be, so it's important that we both go and share about our experience." You can help normalize meeting with a therapist by reminding them of the other times they have talked with a professional about an issue they are experiencing, such as a pediatrician, dentist, eye doctor, or teacher.
With older kids and teens, it can be helpful to ask them if they are satisfied with how things are going right now, and you can point to areas of concern, such as school, friendships, anxiety, or parent-child relating. There is likely a part of them that is not satisfied and wants something to change, and perhaps you have tried to make changes and it's not working, and so in that case, perhaps it's time to ask a professional for new things to try, and to better understand what is really going on.
You can ask them what they already think it means to "go to therapy," and help to reframe their ideas about what it means and what you do in therapy. Yes, we talk in therapy, but we can also make therapy interactive and have a chance to talk about things relevant to them.
Therapy is not just having someone tell them what Mom or Dad wants them to hear. Therapy is about finding ways to grow or heal in order to be healthier emotionally, relationally, or in other areas.