There is a distinction between accessing QF lounges based on oneworld rules (sapphire) and accessing as a Qantas customer.
Under oneworld rules, 1 guest only. Under Qantas rules, your 6yo doesn't count as a guest.
Since you are flying CX, the oneworld rules apply.
However, it has been known for Qantas lounge agents to be lenient when it comes to children but there is no gaurantee. All I can suggest is you ask nicely and happily accept any response.
https://www.qantas.com/au/en/frequen....html#children