Can I limit the use of credit cards by a child account?
This is not an exclusive feature of child accounts, but it is possible to place restrictions on purchasing download versions of software from the official Nintendo website and from Nintendo eShop in the Nintendo Account settings.
Regarding Nintendo 3DS and Wii U, purchasing software from the official Nintendo website requires linkage of a Nintendo Network ID (NNID) with a Nintendo Account. To enable parental controls for the NNID using the system or console to which the NNID is registered.
Please note that if you wish to restrict in-app purchases on a smart device, you will need to check the individual settings for the relevant smart device. Please check the support site for your device for information.