Kidspace Fort and the surrounding park at Chermside are great. Close to the CBD - Roma Street Parklands, with its own [www.weekendnotes.com/parklands-explorer/ Explorer train] are also highly recommended.
Has to be the all abilities playgound recently opened in the Heritage Botanic Gardens. There is a wonderful technological marvel of a toilet nearby for people with a disability. If you would like a tour of Brisbane's CBD geared to folk of all ages with a disability, contact www.brisbanegreeters.com.au