I know precious little about this man. Barker, Sapper C, RE first appears in Chailey Parish Magazine in December 1916. In December 1917 his rank is changed to Driver and it is Barker, Driver C, RE that then appears continuously until July 1919.
He is possibly Charles Barker who appears as a thirteen-year-old on the 1911 census, living with his parents and older brother Ernest Barker (aged 23) at Stanley Cottages, Sheffield Park Station. If it is this man, an enlistment in 1916 for an eighteen-year fits in well with him being called up when he reached the age of eighteen. Unfortunately though, no service record appears to survive for this man and there are too many possibilities on medal index cards to be certain of a positive match with the right man.