It's probably because of her age, not that you missed something. 12-year-olds have a choice of the Club/Lab or Edge, and those attending Edge do not need to be pre-registered. If she wishes to attend the Club/Lab instead, simply go to that open house on embarkation day and speak with the CMs. She may be allowed to attend either kids club and go back-and-forth, or they may require her to make a choice and stick with it for the whole cruise. There have been different reports, and it likely depends on how many children they have of each age group on your particular cruise. So don't worry about pre-registration, you can do it onboard if she wishes to attend the Club/Lab.
Enjoy your cruise!