Carole - many, many thanks, I think you've solved it. So her real name was Maybelle Constance Cynthia Holford. I had looked for the name Cynthia with various surnames but it never occurred to me that it wasn't her first name. And it fits because I know that her father Richard had a sister called Mabel so she was probably named after her, albeit with an American spelling, although she was clearly known as Cynthia.
I am doing this research on behalf of my friend and he said that his aunt, who was the same age as her cousin Cynthia, was always told that she was unofficially adopted by another family and went to live in America but it seems that this was a lie, probably told because when Charlotte died her family did not approve of her relationship with Richard as a married woman (her husband William Wills was still alive). It seems that they then wanted to adopt Cynthia and the migration story was told to them to shut them up. It's a shame because my friend's aunt always wanted to know what happened to her but never found out and she died aged 101 a few years ago.
To answer your questions Charlotte's other two children were elsewhere in 1911 - William was in the army stationed in Regents Park Barracks (although he ends up marrying and briefly living with Richard Holford and his family) while Dorothy's whereabouts are unknown. I was always intrigued by her second name Zillah which is pretty unusual but it didn't help me find her after 1901. Cynthia/Maybelle's father Richard was away at sea in 1911 and worked on-and-off as a ship's steward until 1939 when he retired.
I have looked though the voting registers from the 1920s-1960s and have traced where Richard P Holford lived until his death in 1962. But what is odd is that various other people with the surname Holford live with him including a woman called Mabel (who I assume to be his sister) but it is not clear who they are - their names are Frederick, Masie, Constance and Richard (Jnr). Also I know he married Charlotte's sister Mary Anne Cove in 1938 but for some reason she does not appear on the voting register. Any help would be gratefully received.