Just to add some thought into the mix.
What happens when the family booked flights a couple of months out and the entire flight was almost full at the time, bar some single and double seats (mostly across the aisle from each other).
Had this happen to me booking ANC-DFW travelling a couple of weeks ago. Booked the flight just shy of 2 months out I think and the entire seat map was full except for singles and the rare doubles. Called up and asked if three seats could be allocated together, was told no, asked at the airport, was told no (pretty sure my OWS was known to them if that adds anything to the mix). I'm no longer a kid and can easily handle (sometimes prefer) sitting away from the parents so it wasn't a problem and chose main cabin extra seats in rows 8 and 9 so that I was in the row in front of my parents and my parents were across the aisle from each other. (Fortunately we befriended the couple sitting to the left of my mother so it was all good in the end and quite fun).
The point is sometimes on high demand routes a family can book a reasonable period in advance and still not be able to allocate seats together. Just food for thought. Would you still be annoyed if you were reallocated knowing then?
Second thing I've experienced was Feb last year flying SYD-CNS on DJ. Cannot remember if we could allocate seats at time of booking then or not but distinctly remember allocating seats similar to what was mentioned before but with mum and dad together on the one side a row or two behind me. When we checked in the seats were still like that on our boarding passes. And as we walked through, scanning our boarding passes at the gate, mine beeped and my parents were asked to keep going to not block the gate and I was pulled aside. Fearing the worst, started to panic but was then told that I was reallocated. Questioned why and it was because they wanted to seat a family together. I told them that I was travelling with my parents and wanted to sit near them, they said because I was travelling on a separate PNR they couldn't do anything, and then said besides the plane is full and there are no more seats. Next thing I knew a new boarding pass was printed and I was directed to go down the aerobridge. Ok so there actually was a family together (dad and his two little kids I assumed with the mother across the aisle). Can't remember if I was reallocated an aisle seat or a middle seat (pretty sure it was a middle seat but can't say for certain might be my distaste for DJ/VA after that incident slightly bending my memory so take my allocated seat with a grain of salt) but the seat allocation wasn't the issue, the fact that the plane was too heavy and people were required to be offloaded turned what should've been 3-3.5 hours on a plane into 4-4.5... Couldn't consult with my parents on what to do as they were handing out refunds and discounts for those volunteering to be rerouted through BNE. Tried to get up and go back to my parents but was told to sit down by the FA standing near by...
It's not the end of the world if you get booted, I didn't mind getting moved a year and a half ago. But I can see where people are coming from. TBH if I was to be moved involuntarily now I'd be very annoyed if it was anything more than 5 rows at the gate or if it's getting bumped out of my exit row seat! But if they ask nicely and be transparent and honest about why I'd be happy to either oblige or try and find a suitable compromise. If it happened on the plane, and unless it's for people immediately around me switching for a comparable seat (once offered to move from 13A to 14F so people who were working together could sit together), they can get stuffed. I'd have to agree with JohnK here, if anyone wants to sit together they can either be organised and do it before hand like we have done and pre-allocate seats or inconvenience those down the back and give them a relative upgrade. We've earn't our status the hard way by flying, we deserve some sort of recognition for our dedication and loyalty to manage and maintain status with an airline (loyalty goes both ways) and better seats are a nice way of saying thanks (loyalty goes both ways).
If for any reason you get moved and it inconveniences you (even if you don't have status), raise a fuss with an FA and/or the CSM and demand an explanation until you're satisfied and push for compensation, I'm sure a couple of glasses of sparkling or free booze for the flight would be adequate compo for domestic flights or if it's an intl flight make it known that it's degraded the flying experience of flying with QF and I'm sure some sort of voucher would be coming your way.
Take all that as you wish as it's coming from a comparatively lowly SG with minimal flying experience compared to some others with input here.