how exciting!!

Some places wont let you book too far in advance...and if you do and it requires payment or a deposit, if something goes wrong or your dates change for any reason you may lose that deposit.

Research is good.
Get online and search the 100%purenewzealand web site (their tourisn page) and have a scout for the accom you are interested in...you will want a place that can accommodate kids, day care possibly for a few hours, if they offer shuttle to town or what not.

make sure you find out which resorts are doing what - you dont necessarily want to rock up to one while some snowboarding comp is on (or maybe you do lol)

Dont forget to look at their prices for lift tickets, equipment hire, if it is included in any packges etc, skiing holidays can get exxy very quickly. Also food prices.

NZ is great fun, you will have an awesome time Im sure!
oh and check when NZ school hols are - you will get much cheaper everything if you go outside of school hols.

You and Shel can do your passports now, maybe wait to get Jazz hers until about 3 months before you go - so she still looks like her photo You can apply for a passport online (you fill it out online and print it off to take it to APO for the submission of it with all your docs and photos) They arent cheap, so if you can do yours nowish, then shel's in a couple of months it will help spread out the cost too.

ETA: Passport website
HTH Have fun planning too!