Dilys Golen Ddydd Protheroe was born 18 October 1909 in Merthy to Eleazar Protheroe and Ellen Lewis. She died there in 1977.
She married Henry George Looker in Pontypridd in 1935.
He was born 24 September 1902 in Merthyr and died there in 1981.
In 1939 Dilys and Henry were living at 3 Railway Row, Aberdare, with their daughter Janet who was born 20 Jan 1936 and presumably 1 other child, whose record is currently closed. [All taken from the 1939 Register].
In the 1911 Census Dilys was living with her parents and siblings at 11 Railway Row, Aberdare. The census identifies that her parents had had 10 children, 1 of whom had died. All 9 children still alive were living with them and that included 4 daughters:
Eleanor - Note she was born in 1904 not 1920 as you state
There is a possibility that more children were born after the 1911 census. The following Protheroe girls’ births were registered in Merthyr Tydfil (and whose mother’s maiden name was Lewis):
I have only looked up to 1917, at which point Dilys’s mother would have been 45.
If you search the birth index for a Protheroe child born to a Protheroe mother between 1924-1935 (I guessed that she would be no younger than 15) then there are only 8 possibilities, and in particular there are only 2 in Merthyr Tydfil.
Ceiriog born 1931 July Quarter Vol 11A Page 1077, and Elwyn born 1931 Dec Quarter Vol 11A Page 1010.
The only way you will know for certain which one is hers is to buy the birth certificate, anything else would be guesswork. (Although I am Welsh, I have never come across the name “Ceiriog” before, so I wasn’t certain which gender it is, but he married Iris Hayward in 1954). If Elwyn is the child that you are looking for its possible that his surname was changed because I can’t see any other records for him other than his birth registration.