Hello Mavis and welcome to the British Genealogy Forums.
I cannot help you with how old Sarah was when she went to St Joseph's Industrial School or the years she spent in France. However, I would like to help with other research if I can.
My findings so far tell me Sarah had a sister Margaret Ann born 1880 and a brother Joseph born 1887. I found Sarah and Joseph's birth registrations on the GRO index which gives mother's maiden name unless mother wasn't married. You can search the GRO indexes for free but must first register.
https://www.gro.gov.uk/gro/content/c...ates/login.asp It seems Sarah's mother was unmarried when the children were born as mother's maiden name is not entered on their records. I haven't found Margaret's record in the GRO index, perhaps the GRO have missed or she has been mistranscribed, I have however found her registration on FreeBMD.
FreeBMD: Births Mar 1880
Wigmore Margaret Ann Prestwich 8d 436
WIGMORE, SARAH -
GRO Reference: 1883 M Quarter in PRESTWICH Volume 08D Page 413
WIGMORE, JOSEPH -
GRO Reference: 1887 S Quarter in MANCHESTER Volume 08D Page 241
It appears their mother's name was Julia and her mother's name Bridget.
1881, RG11; Piece: 3998; Folio: 55; Page: 28;
Joseph Smith 27, Manchester
Julia Wigmore, 26, boarder, hawker, Manchester
Bridget Wigmore 76, boarder, hawker, Ireland
Margaret Wigmore, 1, boarder, Manchester
Note Julia and her daughter Margaret are boarders in Joseph Smith's house, I, therefore, think this must be them in 1891 all entered as Smith, after 1891 I lose Julia and her son Joseph. Margaret is on the 1911 census with her husband Reuben Gerlinski, as yet I've not found her in 1901.
1891, Manchester RG12; Piece: 3247; Folio: 113; Page: 9
All born Manchester, boarding with Joseph Gavin
Joseph Smith 35, warehouse porter
Julia Smith 34
Margaret Smith 11
Sarah Smith 8
Joseph Smith 3
Marriages Sep 1906, Chorlton 8c 1727
GERLINSKI Reuben/WIGMORE Margaret
Maybe you already have all the above information but in case not I thought it worth posting.