Hi Dawn,
I keep remembering different places..... This thread will rumble for weeks.
Levin motorcamp, just off the main road in Levin is a good place to break the journey between Taupo and Welly. The kid's playground on the main street is awesome, probably the best we found, and the wildlife sanctuary at Owlcatraz is fab. It's easy to find on SH57 at Shannon.
The big campsite at Tahuna Beach in Nelson is good, but make sure you get a pitch to the left of the office. The amenities building on that side is brand new. The ones on the right side aren't so great. It's a short hop from Tahuna to the World of Wearable Art museum - our kids really enjoyed that. It's worth a trip to Kaiteriteri and Golden Bay further along the coast. The massive flying fox just outside Motueka is awesome. We did it in January and the kids still talk about it. Don't sit in the front seat unless you want to get wet though!
Christchurch has got heaps of attractions. Obvious ones for the kids are the Gondola ride into the Port Hills, Orana Wildlife Park on McCleans Island Road, and the Antarctic Centre which is next to the airport.
Any ideas where you're going to settle once you finish your tour?