Better go to Sunshine Coast or Gold Coast. Brisbane can be travelled within 2 days e.g go to Mt.cook for outlook and sight seeing along the river, or southbank, some parks...etc
Sunshine Coast got pineapple field, macadamia nuts factory, aquatic museum when driving north, can play for a week.
Can I ask what the temperature will be like in Sydney n Brisbane in July or August? I'm planning to take my one year old girl there this summer, but dun wanna be so cold!