This will be handled at check-in or on board. It's probably easier to do on board frankly. Luckily it's an easy swap since they should be able to find a family with older children who would be very happy with row 11. I would bring it to the attention of the IFL, just in case they are so busy with a full flight that they don't spot it. Just one of the perils of curtain roulette, I'm afraid, the passenger rarely wins that one, BA are so keen to get CE revenue that the curtain would have gone back further, but the numbers wouldn't allow it.