BA uses Theoretical Seating - which has its own thread - so family groups are usually kept together one way or the other. If your children are under 12 then call BA when you are in the window of 3 down to 2 days before departure, not before and definitely not after that window, and they should be able to ensure you are OK for seating and may even give you some options unofficially.

Aircraft changes on the day will happen too, I think it's something like 25% of LHR departures have an aircraft swap within 24 hours of departure, though usually of the same Type. The A350 is a very nice aircraft and particularly so in WTP, but if it doesn't work out this time, at some point it will. Second only to the A380 in my personal view, which is also a YVR route from time to time.