This is the itinerary we have decided for our two-week road trip in the South Island of New Zealand: at this time Oliver is 5 years old and Emily is 3.
Staying in the car for many hours is not easy even for children like Oliver and Emily who are used to traveling: I think trains and busses are much easier options. BUT to get around New Zealand the only options are the car or the camper van, which we decided to avoid this time, because it's winter and we'd like to have our first camper van experience in spring/summer (and we did, please check out the related posts!).
To help the kids, before leaving I prepared a map with the stops and for each one I chose a representative photo of what we'll see (I chose what I know they'll like the best, but there is a lot more).

And this is the itinerary that Alex prepared with all the stops and the options in every place (including museums, wildlife, cafes etc that were recommended online). We know we won't do everything, but it's great to have options already picked out when you're on the road: we'll do as much as we can as long as everybody's happy.
We're leaving on July 24th and coming back August 10th (2020). Please check out the article "20-day road trip in New Zealand South Island with kids" in the recommended posts to see what we actually managed to do.
Itinerary
As you can see, everyday there's something for the kids, but also for mommy and daddy: some trips might be more difficult than others (I'm thinking Tekapo Adventure Tour and 3h Hooker Valley trek, for example), but we'll make the most of them.
Fri, Jul 24
Christchurch (1)
Get car
The Grange B&B
Margaret Mahy Playground
Sat, Jul 25
Drive to (2) Geraldine (2:00h) (10-12)
Take the scenic highway
Check out big playground
Drive to (3) Fairlie (0:37h) (13-14)
Have lunch at Fairlie Bakehouse
Check in to Airbnb
Farm tour
Sun, Jul 26
Wake up in Fairlie (3)
Drive to (4) Lake Tekapo (0:30h)
Tekapo Adventure Tour (10am)
Lunch (Greedy Cow Cafe)
Snow tubing
Stay overnight
Mon, Jul 27
Wake up in Tekapo (4)
Drive to (5) Mt Cook Village (1:40h) (10)
Stop at Lake Pukaki
Check into Hotel Hermitage
Hooker Valley Trek (3:00h loop)
Stay overnight
Tue, Jul 28
Wake up in Mt Cook (5)
Drive to (6) Wanaka (2:30h) (13-14)
Puzzling World
Drive to (7) Queenstown (1:00h)
Check into Airbnb
Walk around
Stay overnight
Wed, Jul 29
Wake up in Queenstown (7)
Go on the steamship cruise (1:30h)
Ice skating
Stay overnight
Thu, Jul 30
Wake up in Queenstown (7)
Drive to (8) Glenorchy and road to paradise (1:00h x 2)
Ski lesson?
Stay overnight
Fri, Jul 31
Wake up in Queenstown (7)
Drive to (9)Waikawa (3:30h)
Slope Point
Lunch at Blue Cod Caravan in Waikawa
Check in to Curio Bay Airbnb
Sat, Aug 1
Wake up in Waikawa (9)
Walk around
Relax
Sun, Aug 2
Wake up in Waikawa (9)
Drive to (10) Kaka Point (1:30h)
Nugget Point Light house (1:00h)
Sea lions at Surat Bay
Penguins everywhere
Drive to (11) Dunedin (1:30h)
Check into hotel
Penguin place at 3:45pm
Dinosaur playground
Baldwin street
Larnach Castle
Relax
Stay overnight
Mon, Aug 3
Wake up in Dunedin (11)
Orokonui Ecosanctuary
Albatross
Tunnel Beach
Okia Reserve
Drive to (12) Oamaru (1:30h)
Shag point lookout
Moeraki boulders
Katiki Lighthouse
Check into hotel
Steampunk Playground
See penguins in the evening
Relax
Stay overnight
Tue, Aug 4
Wake up in Oamaru (12)
See wallabies waimate.org.nz
Drive to Christchurch (3:20h)
Stop in Timaru for lunch (13)
Check into hotel in (14) Christchurch (15:00)
Playground
Quake City
Dinner
Stay overnight
Wed, Aug 5
Wake up in Christchurch (14)
Drive to (15) Akaroa (1:00h)
Akaroa Dolphins (2:30h)
Lunch
Drive back to Chch (1:00h)
Stay overnight
Thu, Aug 6
Wake up in Christchurch (14)
Drive to (16) Kaikoura (2:30h)
Whale watching (at 12:30)
Check into Airbnb
Walk around
Encounter Café (kids corner)
Winter paddling
Stay overnight
Fri, Aug 7
Wake up in Kaikoura (16)
Drive to (17) Blenheim (1:45h)
Stop at Ohau to see seals
See WW1 airplanes
Drive to (18) Picton (0:25h)
Monkey Bay
Check into hotel
Stay overnight
Sat, Aug 8
Wake up in Picton (18)
Ferry to (19) Wellington @ 9:30 (3:35h)
Check into hotel
Walk around
Stay overnight
Sun, Aug 9
Wake up in Wellington (19)
Hannahs Laneway
Cuba Street
Cable car
Stay overnight
Mon, Aug 10
Wellington (19)
Zealandia
Return car (5pm)
Flight to Tauranga > (20) home (Home right now is Brightside Farmstay in Papamoa)