Hi BJR, and welcome to cruise-chat.com!
I doubt that your 12-year old daughter will be able to join the teen club. All cruise lines are usually pretty strict about this for liability reasons. You can certainly ask when you board, but be prepared for them to say no.
If it helps, I've visited N. Jewel, and the facilities for kids of all ages are superb. And, if you are going during school break, there will be lots of kids on board, so both of your daughters should be able to make new friends in their age groups.