You can get lost in the State Library !!!! It is so big and it has a free area for young children called The Corner and another area called The Parlour which has old-fashioned games for older kids. Not far from the library you can't miss the Queensland Art Gallery and the Gallery of Modern Art which offer regular programs aimed at children and young adults especially during the holiday.
And then why not have a walk around South Bank. You can easily have a look around, let your kids play and maybe have a ride on the Fairy Wheel.
That's what you can find walking around South Bank!!
If then you are tired to walk around stop by the public library in Brisbane Square, at the high end of the Queen Street Mall. Here there is an indoor play area for small children and plenty of books for you to read.
Talking about parks: there is the Roma Street Parklands and the City Botanic Gardens both with play grounds, picnic areas and plenty of interesting areas for children to explore (no admission fee)
There are many things you can do with children here...so just explore and have fun!!!