For children under 2, travelling as an infant they don't usually come under your normal baggage allowance. You can take them, port-a-cots etc as additional items.

When traveling with older kids, they do become part of you checked in baggage allowance so I used to plan to send it in lieu of a suitcase for one family member. Although when we travelled last year, the staffer/supervisor at qantas check in told me that she has never charged anyone for a pram for a child under 4. It is their discretion though, and she said most of their supervisors would allow it go the same as an infant.
I didn't know this and flew recently with 2 kids over 2 and we had car seats and pram on top of our baggage allowance without issues. (I just assumed they'd allow it) it is probably best to check before you fly (and then check again when you check in)