They flew on different flights. Harry and Charles arrived a day or so before William. William had Tiggy with him on the other plane.
They had to actually change the law in 1983 when Diana insisted on taking William to Australia and so the age was given as 12 and they stuck to that until the QM's death when they made an exception.
Does that mean that by that age William knew that he and his father both were future kings? I always wonder when royal children come to the realization of just WHO they are.