The earthquake.

The entire city must still be healing from the loss and trauma. I hope the reconstruction efforts are going well.
We don't live in a thriving metropolis either but we do live in a middle-size university town with an "okay" art scene including an orchestra, some theaters, etc much like CHCH. You have two young children who seem to be developing at an accelerated rate. As they get older, being able to find mentors, tutors, peers, instructors, etc in CHCH might make a difference.
Until I was 6, I was raised in the country side and it was a wonderful experience and I wish I could give DD the same type of childhood but we'd have a hard time giving up her extracurricular activities. We adore her music teacher and DD still thinks she's going to be a ballerina (then a scientist). Of course, if we move to a rural town in NZ, I'm sure DD will learn to fish, golf, ride, etc and make a whole new life for herself. In a way, you can't go wrong either way. School might not quite be a great fit no matter where you go but I think a school with only 20 kids could actually work IF the teacher(s) has everyone on an individual plan.