Each Copper account can have up to 5 linked child accounts. You can add up to 4 additional kids once you have completed setup and opened a bank account for your first child. Simply navigate to Settings --> My Family --> Add a teen in your Copper Parent app.
If your child has already signed up for Copper you can enter their code to link them to your account, if not then just send them an invite by following the steps above. Your child will receive an invite to download the Copper app and create their account.
After your child has completed their registration link you will finalize their account setup by clicking on 'Order Card' and completing those steps, this will be found under Settings --> My Family and on their name icon. Your child's debit card will then be shipped to your address and you'll see their account populate on your Copper home screen.