Draco indeed is Lucius's only son. But not an only child. A little girl by the name of Cherry Malfoy lived under the staircase of Malfoy Manor. The Malfoys disowned her because she was a squib. One sad, sad night, Lucius lost his temper. Using the House Elf Transfiguration spell, Lucius preformed a memory charm and turned her into a house elf, by the name of Dobby. Harry never noticed because the memory charm was a very strong charm that only a powerful wizard or witch like Dumbledore or He-Who-Must-Not-Be-Named could a feast have a chance of undoing. Cherry was older than Draco by eight years. Draco was never told of her because the family didn't want "Dirty filth" as they called it, in their house.