You can hope and wish, for the 330 to operate on your flight, but its not till you get your bums on the 330 and its taken off, and hasn't done a banking turn back to the originating airport, that you can enjoy the row behind the bulkhead in Y.
QF could easily swap "due to operational reasons" from the 330 to the 737!
I had to call them (QF) up 3 times, after I paid for a J seat, and they kept swapping the plane I was chasing from a 330 to a 737 MEL - SYD on a pure Aust dom flight.
Not sure of your status, and whether you can get that first row of Y seatings, right behind J cabin.
And hopefully you will make that 330 and get the bulkhead seatings for the 2 of you.
I have flown the QF A330 from PER - SIN a few years ago, both dep PER late afternoon, getting to SIN about 9.30pm, in Y, and back then, back to PER, it was a day flight.
My experience in a 747 was PER - SYD in Y, 3 abreast, and didn't know better, thought I could/would keep my smallish backpack in the spot under the seat in front of me, nak, never again.
Did 330 in Y from PER - MEL on the 330, was great, left PER about 3pm, and got to MEL about 9.30pm, 2 abreast on the 330 is a very good thing, but its a pity that QF does have to do equipment swaps, for so many reasons, one being the majority of the 330 are going Aust - NZ, so more 737s will be utilised on the Aust trans continental flights, ...
Maybe a weekday flight, has more chances of them keeping a 330 for your planned day of travel, but as we know, they can't "guarantee" types of planes to be/will be used.
The thing about QF9/QF10, with a toddler, is you have to do immi and customs, or quasi immi and customs form filling, ie, your BPs has to have the orange sticker, and you have to go through the immi counter...
First even with online checkin for the dom sector of QF9/QF10, you will still have to present photo id for the both of you, for them to issue the BP with the orange stickers, which is not kept by you, you present it in MEL T2 outgoing immi, keep it with you all the way, and at T3 PER, you surrender the BP with the orange sticker to incoming immi.
If the kid was older, it might be a plus factor to show them how outgoing and incoming Aust immi works, but if they are still too young to understand, its a lot of rushing around, walking very fast from T2 outgoing int (international) security screening, to outgoing immi (no smartgate for the dom sector), then via the myriads of duty free, to the gate, and then same on the way coming into T3 in PER.