Maybe someone with more recent knowledge on the subject could refresh my memory here. (I worked in BA reservations for a number of years many many years ago.) A child is considered an infant up until, but not including, its 2nd birthday, at which point it becomes a child. An infant is not required to have a seat (ticket yes, seat no) but the family may purchase a seat at the child fare if they choose to do so. (On Concorde they had to - infants weren't allowed.) A child (2nd birthday to 12th birthday) must have a seat. From what I remember (and I may be wrong here) if they set off on their journey with an infant, but that infant has its 2nd birthday while they're away and becomes a child, it's the airline's responsibility to provide a seat for its return journey.