If Qantas oversell their flight by 2 seats and everybody checks in, then they need to bump 2 people from that flight and make alternate travel arrangements for those people. 2 people travelling together on the same PNR are harder to rebook than 2 unrelated travellers as you can access alternates with only 1 available seat rather than needing alternates with 2 available seats. Adding a baby to the PNR means you now look for alternates with 2 available seats and bassinet access which is harder again.
I'm not suggesting that bumping one party over the other is the 'better' option, or that travelling with family/baby entitles you to better protection from being bumped, just that in my mind the ease of rebooking the bumped passengers would be a factor in determining who gets bumped, and in that light having complex travel arrangements (like a family with a baby) works in your favour.
Of course you could just as equally replace 'bump' with 'op-up' in the above missive and it's the same message without any implication of entitlement as a family traveller - which is to say that there's a higher probability of the OP with her husband and baby flying on the route they booked in the cabin they booked than any given solo traveller on the same flight.