According to gamespot, parents setting up a Nintendo Network account for your children under the age of 13 will need to pay a one time fee of $0.50.
This acts as proof of consent to create a Nintendo Network account, and is in accordance with the Children's Online Privacy Protection Act (COPPA).
According to Nintendo, credit card information will not be stored on the Wii U. Additionally, because this information is not stored, gamers must re-enter their information to make purchases through the eShop.
Parents and guardians with multiple children will not need to pay the fee again to create additional Nintendo Network accounts. Once the first child is set up, the Parental Control PIN is the only information needed to create additional accounts.
It's a minor annoyance, to be sure, but I think it helps highlight the difference between Nintendo and other platforms. Nintendo actively assumes that there may be kids playing on their system, and thus is proactively compliant with child safety standards.
It is surprising because I hadn't heard of anything like this before. I knew that there was a facebook age limit of 13, perhaps this is the same COPPA regulation at work?
Note that this applies to an online Nintendo Network account. It doesn't apply to user accounts on the system, or Mii's, or anything that is just local.