It appears your grans' recollection may be a little out as it appears Margaret and Alexander actually went to South Africa before coming to Canada.
I found Margaret (44) and Alexander (14) on an outgoing UK passenger list at Find My Past for the Grantully Castle which departed from London on 2 June 1927. Their last address in the UK was 645 Cathcart Road, Crosshills (the same address Robert gave for Margaret on his Form 30). They were destined for Capetown, South Africa.
Next is an incoming UK passenger list for the Guildford Castle with Margaret Sawers (41) and Alexander Sawers (15) whose intended UK address was Farme Castle, Farme Cross, Rutherglen and they arrived in London on 16 Apr 1928 with their departure port being Mombasa, Kenya.
Then the two departed from Greenock and arrived in Quebec on the Laurentic on 21 Oct 1928. Alexander was 15, a motor engineer, born in Lanark and his fare was paid for by B.I.C.A. Margaret was 36, a housewife, born in Glasgow and her fare was paid for by Empire Settlement. Their destinations were listed as B.I.C.A., Montreal and Women's Hostel, Montreal respectively. No mention of a father/husband was made. The person listed as their nearest relative in the country from which they came was a friend, Mr. William Black of Farme Castle, Farme Cross, Rutherglen.
The addresses link the passenger lists together and to Robert Sawers although Margaret looks to have shaved a few years off her age as she got older.
Jacquie
PS
According to Library and Archives Canada, B.I.C.A. stands for British Immigration and Colonization Association and Empire Settlement was:
Under the Empire Settlement Act of 1922, the Canadian government offered assisted passage to young men and women, in the form of loans and reduced fares, to encourage the immigration of farm labourers and domestic workers.
Both were immigration schemes.