The family is a unit of consumption because the capitalist society advertise things and products so that people can buy them. Another point is that children always ask their parents to buy them things so that they can probably fit in with a group of friends etc. This is sometimes called 'Pester power'. Also, the family is forced to believe that they need certain things, when they really don't. This is how the family benefits the capitalist society.
OK Connah, to put it briefly, sociologists use the term 'consumption' to mean things people buy - so I figure you need to think of a list of items and products and services that are aimed at and bought by families - from houses to breakfast cereals!