We did this trip last year, except to London, and it was the flight back that absolutely killed us. The kids slept all the way over, which made economy manageable, but trying to keep them occupied in the cramped confines of coach coming back was an incredible challenge.
We did have an IRROPS coming back last year and were rebooked on Virgin Atlantic out of LHR at the last minute. Are you saying that the only risk would be that there might be a situation in which they would rebook one of us on a particular flight but not the other? In that case, we could just say "no thanks, you've got to keep us together"?