Brisbane Riverwalk is a great way to see the river, and Waterfront Park in Newstead provides the ideal stopping point for families to play and refuel. The park has a spacious playground which is part of the Waterfront development, with hundreds of newly built riverfront apartments.
The children's playground has a maritime theme with a pirate boat to play on, interesting geometric shapes to climb on, swings and various spinning aparatus. There are lots of free BBQs, native trees for shade and grassy space to run around.
The area looks onto the Brisbane River, with some interesting metal sculptures and water features dotted about. French Twist Patisserie is conveniently located on the edge of the parkland - see Clare's great review here.
With plenty of car parking available nearby, I also recommend Waterfront Park as a start or finish point for hiring CityCycle bikes and taking the rather flat Bikeway to the city. Newstead's Waterfront Park is lots of fun - add this one to your list of great spots for kids to play in Brisbane.