Thanks so much everyone. This level of detail is incredibly helpful! For reference, we would be flying out of London to Washington and back to London in Y as there is no miles availability around my dates to use my amex voucher. I would be travelling probably from the 19 to the 29th.
Great points about the weather for connections and also the seating for a group of three. We have flown a reasonable amount but I am still not used to the older child having his own seat.
Ironically the prices are changing so quickly in the last 24 hours that the savings are not so great on connecting flights home any more but still maybe worth it.