You can add a second (or third, fourth, etc.) child to the family account at any time! Simply log in to the family admin account and go to the Member Dashboard.
In the Student tab, you will see a button to add another student. You will be asked what kind of subscription you want with that new account. You do not have to choose the same subscription level for every child account.
You can manage the subscriptions you have independently – pausing or cancelling them at different times as needed.