On an A350, that's also CS, so I'm not sure that keeping the group together really works. But Theoretical Seating, at 3 days to departure, will usually keep you together to some extent (see TS thread). TS generally works fairly well, we don't get as many "we've been moved" / "we've been split up" complaints, compared to before TS. If you look at the seat map on ExpertFlyer just before and after T-72 you can probably work out what is going to happen, and it's ok at T-48 (but no later) to ring BA to check that the seats for the children are OK. Either it's important or it's not, so you pay or you don't, but Ruby/Bronze doesn't require much oneworld flying and that would normally help out here.