You wouldn't expect a kid's playground in the middle of Sydney CBD, but there is one in Darling Harbour and it's quite big. And the best of all it's free!
The playground is located between Chinese garden and IMAX. There is a massive slide, flying fox and water fountains. To enjoy the water features, you should visit there on a warm day with a swimming costume or a spare clothes. My son had such a great time, and he refused to go home, so be prepared to stay there for a while. Just for your information, there is a McDonald's (you can get a Happy meal for your child) and a toilet.