I thought only living people were listed in the census

Well .... I guess that's not always true as can be seen here in an excerpt from the 1911 UK census.  

We likely would never have known of these young children ... so thanks Edward or Eleanor, whoever talked to the census taker.

Rosina Hankinson of Garston/Liverpool: Her Name was Key to Finding a Missing Person

The Hankinson family of Liverpool, England had 9 children, the oldest of whom was William born in 1865. He was with his parents in the census of 1881, aged 16 and a plumber. I could not find him 10 years later in the 1891 census -- not with the family, not alone.