Coming from a Nintendo customer service rep...
Thanks to Eduardo for the heads up!
Thank you for contacting us. To answer your question, 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 (12 and under). To verify that a parent or guardian has granted consent, they will need to provide a valid credit card number when setting up a Nintendo Network Account for the child.
A one-time fee of $0.50 is charged to verify that a parent or guardian has provided consent. This one-time payment is a commonly used and accepted parental control method required so that parents have the chance to see the payment on their credit card statement, and to alert Nintendo if consent was improperly provided.
To answer your second question, a fee is not needed to set up subsequent child accounts on the system. Once the first child account has been set up, the Parental Control PIN is all that is required to set up additional child accounts. I've included a link below with more information on COPPA and Wii U Parental Controls:
Again, thank you for writing. I hope you find the information I have provided helpful.