Can I Add an Existing Nintendo Account to My Parent/Guardian Account?
Existing Nintendo Accounts for users aged 16 and older can be added to a parent/guardian Nintendo Account as a supervised account. The parent/guardian can restrict Nintendo eShop purchases, but cannot manage any other settings for the Nintendo Account (Other types of Parental Controls can be managed on-device and/or via a dedicated smart device app).
For information on adding a supervised account, click here.