Not sure of your budget but Gold Coast has quite a number of family friendly accommodation options that may mean you can enjoy some family fun without going out and spending a fortune at the theme parks - worth checking out online perhaps ?

We've stayed at a few different places from Crown Towers resort apartments with a great pool area, to the dedicated kids resort Paradise Resort, which was a bit older and tired when we stayed there but awesome for entertaining the kids - we also stumbled across a fantastic caravan park on the way in once, not sure what it was called but it was around Coolangatta, we only stayed there for one night in a cabin but the kids loved it

In terms of theme parks, we've taken our kids to the Gold Coast a few times over various age range spans and found they enjoy Seaworld at pretty much any age, Dreamworld has decent younger kid areas as well as great older kid/adult rides - then there's the Currumbin bird sanctuary, which my younger ones really enjoyed - there's also a decent public playground along the foreshore between Southport & Surfers.

Brisbane is a little more challenging for kids of that age, and I can't say I've really tried to entertain my kids much there - only thing I have done with them is South Bank and the markets ... maybe someone local can offer more advice on this, as I don't have much to offer sorry !