When we had OzFest in Surfer's Paradise last May, we visited the Carrumbin Wildlife Sanctuary a few miles outside town. Take a look at the photos I took there and see what they offer. The place is big enough that they have a small train you can ride around on. Plenty of beach right there in Surfer's for the kids, too, without having to drive off somewhere. Yes, it is touristy, and you'd probably want to avoid the place during school holidays, but might be perfect for the kids.
Carrumbin Wildlife Sanctuary PHOTOS
Surfer's Paradise PHOTOS
I went to the Lone Pine Koala Sanctuary a dozen or more years back and about all I can recall is walking in a pen with kangaroos, so can't be much help there.
One other place to consider north of Brisbane, on the Sunshine Coast, is Australia Zoo, associated with the late Steve Irwin. Don't know if his TV programs are still airing in the U.S, or whether your kids might be familiar with him, but just something to toss out there if they are. I've passed by it a bunch by both bus and train over the years but have never gone in. There's also an aquarium nearby in Mooloolaba.
http://www.australiazoo.com.au/
https://www.underwaterworld.com.au/