My daughter and niece (14 and 12) went to a few of the events for the tweeners, we were on the Inspiration in late May. I would recommend you register your 12 yr old, at that age they can sign in and out as they please. My girls quickly found a few kids their own age.
I knew they were always either 1) in the pool, 2) On the slide (on the way to the pool), or 3) In the Lido restaurant stuffing their faces with ice cream. Honorable mention (#4) is terrorizing boys ages 12-14...
If your kid wants to participate in the activities, you'll need to register. But if there's nothing that seems all that interesting, it's not like it manditory or anything.
I haven't cruised the Freedom, but I've been on the Conquest twice. They are sisters, and I think you'll have a great time.