Nintendo Network users on the Wii U looking to register a child under 13 must pay a one-time fee of $.50, according to an entry on the system's online support page.
Located in the General Info tab, under "Is the Nintendo Network free?", the post states that "in accordance with the Child Online Privacy Protection Act (COPPA), a parent or guardian must provide consent to set up a Nintendo Network Account for children under age 13."
In order to confirm guardian consent, the post reads, a credit card must be provided. The user will be charged a one-time bill of $.50 as validation, with additional minor registrations at no cost. Card information will not be stored on the system.