Looking at the numbers (and this is admittedly akin to FT witchcraft in terms of scientific rigour) I don't think WT on the outbound service is full. It's G8 and L2 which suggests quite a few spare seats, assuming it has been recalibrated for the lost of 27 WT seats on the 787-9. So I'm wondering if you get to the airport in good time whether you can persuade an agent to come up with a seat configuration which has an extra seat in the block for you. For example row 41 is completely clear, and the curvature of the aircraft and lack of a window makes 41A and 41K a bit of a poor seat for adults. But I doubt it would be noticeable to children. There is a family check-in area in zone A (far North).