Perth!! You could head down south (cooler but still lovely), or go up north (still warm, swim weather etc).

Otherwise, what about Club Med or similar on a Queensland island? We went to Club Med just off Singapore a couple of years ago when the girls were 2.5 and 3.5 and it was fantastic. They really do the family thing well, and they have a kids club so if you're lucky you can have some time to yourselves. My kids went once - raved about how much fun it was, nearly turned themselves inside out waving to and hugging the carers when we saw them wandering around the resort but flat out refused to go back! Anyway, with all inclusive costs and the many offers they seem to have consistently throughout the year it makes for a pretty reasonable, very relaxing holiday.