At Ancestry, the birth registration for Adam and Janet's daughter has her name as Susannah Henderson. Janet's name is listed as Jennet Carlisle Gibson although I don't think that's going to be much help finding her birth as there were no births under that full name (Janet/Jennet/Jane and other variations) registered in Scotland.
Also at Ancestry, on the 1901 Scotland census in Kirkmahoe, Dumfries there is an entry for Janet Henderson, 38, born: Muirkirk, Ayrs and her daughter Susannah McN Henderson, 12, born: Canada. I didn't see an entry for Robert. You can view the image at ScotlandsPeople -
http://www.scotlandspeople.gov.uk/Also at ScotlandsPeople, there is a marriage registration dated 1911 for Susannah M'Kay Henderson to Walter Scott Halliday in Annan, Dumfries (I didn't view the image so can't say with absolute certainty this is your Susan/Susannah).
At FindMyPast there is an entry in the Outgoing UK Passenger Lists database for Mrs. Halliday, age 25 and her son Walter Halliday, age 1 departing for New Zealand aboard the Corinthic on 10 Sep 1913. There are two registrations (can't be viewed online) at ScotlandsPeople for births of boys named Walter Halliday in Dumfries - one in Annan (where Susannah was married) and one in Hoddom. There was a Walter Halliday (travelling alone) who departed for New Zealand on 4 Dec 1912 aboard the Corinthic.
In case you don't have it, Adam Henderson and Janet Gibson were listed one after the other on the passenger list of the Norwegian which arrived in Quebec on 1 Nov 1886. Here's a link to the image (they are listed near the bottom of the page):
http://data2.collectionscanada.gc.ca/e/e142/e003546608.jpg