William Shelley

By June 1816, Mary Shelley had a child (AUDIO: The Witch from the Well) named William. (TV: The Haunting of Villa Diodati)

Behind the scenes
In the real world, Mary Shelley had five children by Percy Shelley, but only one survived past infancy:
 * Clara, born in February 1815, survived only eight days.
 * William, born in January 1816, lived for 3.5 years, to June 1819.
 * Clara Everina, born in September 1817, survived 1 year, to September 1818.
 * lived to the age of 70, but he was born in November 1819.
 * An unborn child, who died in June 1822, due to a miscarriage.

At the time Mary met the Eighth Doctor, William Shelley would have been her only child, born five months prior. William died three years later in Rome (on 7 June 1819), of either malaria, cholera or typhus. The Haunting of Villa Diodati later explicitly named this child William, and included an appearance by Mary's baby.