Montréal is notorious for equipment changes, and the switch between 787-8 and-9 is very common, and I think they've seen some -10 as well, and could easily go to 777 too. It's a profitable route for BA, but it doesn't always have high loads - these two factors are independent of each other. If your granddaughters are under 12 then they will need to be proximate to an adult. Appreciating this is a nuisance - for which you should recover your seat reservation fee - I hope you have a good time in London. You are on G-ZBJD, a 788, they are on last call in London at gate B33 at the moment, so you are looking good for an on time departure from Canada.