Pre-registering will cut down the amount of time you will need to spend at the terminal or on the ship getting her a magic band (only needed for Club/Lab), but you will still need to go through a line there to complete registration.
Pre-registering for Edge will put her name on the Edge roster, but there is no need to go through the Youth Activities registration lines if all she wants to do is Edge. There is no controlled access at Edge, the kids are allowed to come and go as they please.
The Club/Lab requires all kids to wear a magic band and there is a formal check-in/check-out process. Kids can be given self check-in/check-out privileges at their parents discretion.