Why Do Children Under 16 Have to Join a Parent/Guardian’s Account?
In compliance with European laws, Nintendo does not knowingly collect personal information from children. If you are a parent or guardian that wishes to allow your child access to a Nintendo Account and/or My Nintendo, you will need to create your own Nintendo Account to which you may then associate your child's account.
Once you complete the registration process, you can add Nintendo Accounts for up to 5 children to your account, and manage the settings for each child that you add.