I took scuba lessons at 14, but I could not be officially "certified" until 15. I don't know if a 12 y/o can do the discover scuba class or not. Check with Carnival. As far as being experienced swimmers, don't worry at all. My mom took the lessons with me and she can hardly swim a stroke. I think if either of you is claustrophobic at all, that poses more of a problem than being poor swimmers.
Go ahead and do it to see if you like it and think it would be something you would do again. Where I live, the cert course isn't expensive, but the checkout dive is in pensacola, FL, which I hear is crappy and doesn't do justice to the scuba experience for a first time diver.