What would you do if your child picked a different name for themselves?

Kind of an interesting story here. A family friend has a 13 year old daughter. The girls name is Emma. When she was very young (I think around 3 years old) she started calling herself Clarina. I believe she heard the name from a book and just latched onto it. Her parents would insist on calling her Emma but she would still call herself Clarina. When people would ask her name, she’d say Clarina. She writes that as her name on all paperwork unless she legally has to write her legal name. She still does this. So what would you do if you were her parents? Her parents call her Clarina now and so does everyone else. But I’m wondering what others would do and if you’d consider legally changing the name (if possible where you live).