Yes, you will end up with at least one adult plus one child, even if that has to be done on board the aircraft. Theoretical Seating (see separate thread) will normally keep you together anyway, but what you should do is when you are 3 days from departure, and no later than 2 days from departure, call up BA and see if the agent can double check that everything is ok and that the computer has done the right things. If you did nothing then the Inflight Lead or Manager (silver tie or cravate) will help you to arrange things on the aircraft.