Brisbane is really very child-friendly. Southbank has Streets Beach...which is a public swimming pool with sand for the kids to play in. You can take them to Lone Pine Koala Sanctuary for a wildlife experience - they'll love the koalas and other animals for sure. Something my kids like is taking a ride on the rivercat along the Brisbane River. Theres also the Planetairium at Mt Cootha Botanical Gardens. They have shows there especially for kids and while you're there you can have a picnic by the lake in the Botanical gardens - take some stale bread with you so your kids can feed the ducks! I hope this helps
Things to do with kids in Brisbane when visiting