I didn't know that BA offers an all-you-can-eat buffet in J.

Seriously, the OP is not trying to get two meals out of BA. They are trying to switch off which family member is sitting alone. I don't see this as a big deal. I doubt that the child is going to be pounding back the gin and tonics, so the argument that they are switching off so that everyone can get sloshed on free booze in J seems a bit far fetched. I really cannot see why BA or any other pax should care.