It Costs $0.50 To Register A Minor On The Wii U
The small fee technically makes the Wii U's online services not entirely free, but it's a way to confirm that a guardian is truly approving registration instead of a lying 12 year old.
The fee is there so that Nintendo is compliant with COPPA, the Children's Online Privacy Protection Act. Parent's only have to pay the fee once if they are registering multiple children under the age of 13, and the credit card information is not stored on the console or online.
You can head to the Wii U's support page to find out more the fee.