How lovely to have that longer stretch of time!
Personally, I would not go to Aus/NZ. First because it’s their winter and second because they are both amazing but your kids are too young to remember it. I would save that for another time.
I would recommend interrailing in Europe. I know a few people who have done this recently with kids. The interrail ticket makes the travel more affordable and convenient. You can cover a lot in the time you have.
if you enjoy the hotter weather I’d suggest Spain, France, Switzerland, Austria, Italy. Or, for a little cooler, Germany, Denmark, Norway, Sweden, Finland. Lots of other options/routes. You could choose six destinations to base for a week each, or (my choice), 12 destinations for half a week each. Mix it up - some cities, some seaside, some historic, some fun (legoland?).
I’m jealous! You’ll have a wonderful time.