To help manage a Regpack family system, admin are given extra tools that regular admin are not- family system admins have the ability to add additional children to a family in the back end of the system.
In order to do this, first access the parent's side menu (the side menu's icon is a triangle inside of a green circle, and it appears to the far right of the frame when you mouse over a user).
Once the menu window is open, find and click the "Create child / sub - unit" link under the "Users" category. The child's registration will be created instantly.
At this point you can either fill out their information, or instruct the registrant to complete the child's new registration.