Of course there are lots of variables, but my experience with my own children, (which is fading into memory, as they're all now adults) was:
* We started with an overnight tent stay on Mt Wellington (towards Thark Ridge). We cooked over a fire and slept in a tent. The kids were probably 5 yo and up.
* We took them on a few long (for them) day walks (such as South Cape Bay, or Dove Lake to Kitchen Hut). We got them to carry a backpack, just to get them used to it.
* Our first "wilderness" adventure was a trip into the Walls of Jerusalem with the older two (aged ca. 8 and 11). We got a good forecast, and spent two nights in there. They've never forgotten it - so much so that the youngest (who was 5 yo, and missed out) begged me to take her in there some years later. And last summer she went back, this time to show her husband that wonderful place

* A less "fun" experience was when my wife and I took all three children up to Shadow Lake. Our food took hours to cook because I'd frozen it, the children bickered incessantly, and we were flooded out by an enormous thunderstorm.
So it hasn't been all sweetness and light ... but we're all still walking. And we can now laugh about Shadow Lake

What do others reckon?
cheers
Peter