General Nintendo Accounts and Nintendo Accounts for children are different in the following ways:
An e-mail address cannot be registered to an account for a child. Any e-mail notifications are sent to the e-mail account registered within the parent/guardian's Nintendo Account.
Nintendo Accounts for children can be created for users age 0 through 17. General Nintendo Accounts can only be created by users age 13 and older who have an e-mail account.
Some My Nintendo missions can only be completed by users with a Nintendo Account.
Nintendo Accounts for children can be linked to a Nintendo Network ID only. They cannot be linked to Google accounts or Apple IDs.
All Nintendo Accounts created for children aged 12 and under are automatically set up as a supervised account in the Nintendo Account family group.
Other than these points, they function almost identically.