Wow that sounds like an awesome trip--especially for the 5th grader!
My immediate thoughts--since you're also a teacher, I know you have plans already on what to teach-- but take a quick look at the Semester at Sea web site, which is a US-based college semester abroad, through the U of Virginia (I did it in 1989 (!) as a sophomore in college). I like that you take 4 college courses as you travel around the world: 1 is called "Core", which is kind of a survey class, which can encompass anything; the other 3 are focus courses, like "gender in developing countries", astronomy, cultural geography, race relations, economics, etc--and in most of them, you study about the country you're about to go to first in terms of gender, race, economy, an then, when you're there, you do a short project observing what you just studied, to see if it's true on the ground, or if your theories are correct, etc. Obviously, 5th grade is not college, but having a focus and doing comparative studies about the same topic in different countries might help give him some idea of differences he can focus on while on this amazing trip!
I'm not sure if you already have an educational plan figured out, but for me (also a teacher), it is always nice to at least have a framework for the year/month that is consistent, where he can focus on a few areas--maybe science (geology/geography of each country), social studies (race/class/gender), math (econ, food prices, earning power), literature/language, etc---
and then of course you can do what you want in between if something is especially interesting/exciting. Maybe he creates an online database for each area and each country, and adds to it with each new country---finally doing a research paper/project at the end comparing/contrasting areas of interest?
Oops--sorry--the teacher in me just rambled. But you get the idea!
As for New Zealand, I prefer the South Island for sightseeing (Fox Glacier, west coast is gorgeous, and Milford Sound has fjords!-maybe take a week and drive around?)--but I was on the North Island last July and it really is great too. Tongariro is gorgeous, and does really look like the mountains from Lord of the Rings (which was filmed there); Napier is a pleasant afternoon of walking around an oddly uniform downtown full of Art-Deco architecture--it was destroyed in the 20s in an earthquake and rebuilt all in Art-Deco; Rotorua is touristy but the Maori culture is most accessible there--and the city museum is worth an hour or two; it used to be the "spa" that 1800s Brits would come to for therapy, before a huge earthquake--which is also explained at the museum.
For geothermal activity, which is all over the Rotorua area (there is steam rising from random places in every corner of town!), check out my video on my facebook page for a place with bubbling mud pools and geysers, maybe 30 min outside of Rotorua. I'll find the name later. Taupo is not as touristy as Rotorua, but is a smaller city; I liked it but not a lot happening--which I also like sometimes.
I didn't go but the Coromandel Peninsula is supposed to be really cool; as are the caves!
Maybe helping with local Maori group in terms of education/nature restoration? There is lots of logging between Rotorua and Taupo which might be of interest too.
More later.
Have a great time planning your adventure, and maybe we'll cross paths this year on your trip!
J