There's a birth for a William INSTANCE 1909 Southampton with corresponding death in 1994- so the surviving child of the three just not at home that night?
(William and Joseph appear to be twins, with a death of a Joseph 1909 in South Stoneham?)
That said, there is a census that could be the above William, living with his father (Mr INSTANCE...), a widower. (Any chance your people didn't marry because she was already married...?)
~~
OK, the father of the above William was Joe Joseph INSTANCE. Perhaps the William on the census wasn't a child of hers, but they looked after William for his widowed father, accidentally put him down on the census when he was at home that night, and crossed him out?
They were neighbours. The DICKSONs lived at no10, INSTANCE at no13.