I had to open this thread to see why it was still being discussed...ironically what is being discussed is why it's still being discussed, 13 pages later! You have love FT sometimes.
Resort to the first few replies and pages for more valuable information. Hopefully the OP has seats together now and maybe one day the complexities around family bookings will be incorporated by BA's IT systems or FLY to recognise this. My sister had a similar situation recently when her 2 young kids were seated away from each other and the parents, even the 2 kids were separated in the booking, so randomly scattered around the cabin, it does seem odd that FLY does this. It was one PNR, all the same family name both under 8 yrs old, in WTP. It took to getting on board and asking to move for it to be rectified. Like I said, shame the IT side can't sort this yet.