Living there, I found I used a 12 90% of the time (90kg, with a twintip). The rest of the time I was mostly on a 17, with very rare sessions on a 10.
If you're taking a surfboard, your 8 and 12 will probably do, unless you're a bit of a fatty.
Faber - it's quite expensive, but very safe and very family friendly. Unless things have changed significantly in the last couple of years, the most popular kite beach (Silver Rock/Silver Sands) can be stressful with small kids - it isn't a very big launch area, it gets crowded, and it gets a bit nuts when there are lessons going on. Not a problem provided that someone can keep a grip on the small people. Long beach (a little to the Northeast) has stacks of space, but has no facilities and (sometimes) a pretty brutal shore break. The West coast (where most of the popular tourist beaches are) is more or less unkiteable for most of the year.