Here is something dumb I discovered the hard way. Children under age of 13 can have club nintendo accounts. They can log into website, enter their own codes. Children under age of 13 can also have their own NNID and buy games on the eshop. However, any NNID account under age of 13 cannot link club nintendo account to the eshop.
So, a kid can buy a game at retail and enter the code to earn coins, but a kid cannot earn coins for any digital purchases. I'm sure this is somehow related to the bullshit US laws regarding children under 13 and "privacy".