In order to install some of the free software packages on your or your child’s Nintendo DS you are required to link a Nintendo Network ID which can be a cumbersome process and will require a credit card to verify you are an adult. The question often arrises on whether you should create yourself an account or create your child one. The answer I will give you is that you should create an account for your child but retain the credentials for it (as well as setting up parental controls with a password and NOT storing your credit card information and instead entering it with each transaction as an extra measure of not having your child accidently spend money if they were to figure out your pin number). When creating an account Nintendo will make a one time charge of 50 cents to verify that you are a consenting adult to your childs usage.
The advantage of linking a network ID should be that you can transfer and/or re-download titles for your DS that you have previously downloaded. It also will allow you to share your account between your DS/3DS and your WiiU if you happen to own both systems.