According to Library and Archives Canada "Between 1869 and the late 1930s, over 100,000 juvenile migrants were sent to Canada from the British Isles during the child emigration movement."
According to an abstract for a presentation at OGS conference 2016 "Ten percent of the current Canadian population is descended from the up to 120,000 British children sent to Canada . . ."
The figure of 10%, or thereabouts, is frequently quoted although I've never had the basis satisfactorily explained.
Those figures depend on how one defines a child. Most estimates appear to be based on names and statistics recorded for those who came to Canada under programs operated by an agency, the most notable of which was Barnardos.
A common dictionary definition defines childhood as extending from infancy to puberty. A definition in terms of age is more administratively convenient with ages 14 and later 16 being applied consistent with the school leaving age. There is confusion between the state of being a child and a minor still legally under the care of a parent or guardian.
Library and Archives Canada's Home Child Records database at http://goo.gl/7YosQr givess surname, given name(s), age, ship and year of arrival. There are limitations on the number of results you can view for a search. To explore how many would legally be considered children under various definitions I examined a sample of immigrants with last name Smith who arrived between 1869 and 1899. There were 769 in total with ages given for 746 of them. Results are shown in the table.
This indicates that 39% of young immigrants in the LAC database arriving in the period were over the then current school leaving age of 14.
As a group the term "young immigrants" as used by Marjorie Kohli as the sub-title to her book The Golden Bridge, is more appropriate than children.