Flying QF business class in Australia will get domestic departure lounge access. Your 8 year old son will be allowed access with you.

In SYD you have to make your way from the international terminal to the domestic terminal. In the international terminal follow the QF domestic transfer signs. Turn right exiting immigration, with your checked bags, to the QF domestic check in area in the international terminal. Drop off your checked bags here then get the QF airside bus over to domestic.