I couldn't find William or Mary in the 1911 census, nor Mary in the 1921. I did find an 11 November 1920 border crossing record for Mary that has an interesting twist - she was travelling with her son, Clifford Allchurch, born in Woodstock, Ontario about 1912. Mary says she's a widow, living in Grimsby Beach, her closest relative in Canada is her sister, Mrs. Annie George in St. Catherines, and she's driving to Jacksonville, Florida to visit a friend, James Ford. https://search.ancestry.com/cgi-bin/sse.dll?ti=0&indiv=t...https://search.ancestry.com/cgi-bin/sse.dll?ti=0&indiv=t...
I could find not find a birth record for Clifford. He would have only been about 10 when Mary died so he either went to a relative or an orphanage.
I would try getting a copy of Mary's death record and possibly Clifford's birth record.