Ah, well that's a sad story. The term they're using is actually "bound girl." It's a form of what has been called, in other places, "indentured service." As a way of dealing with poverty in Britain, poor children were delivered to homes in Canada — especially Ontario. These were sometimes orphans, but just as often children from families who had too many children, simply couldn't support them all, and had abandoned one or more of their children or let them be housed in a British charitable organization.
From that point, the children were legally "bound" to do service in the Canadian home until the age of 21. It would be fair to say they were "sold", as the receiving families did pay a fee for them. The receiving home was obliged to house and feed and "educate" them. Of course, the quality of care could vary widely, and standards of "education" were very loose. After all, in society's eyes, being trained to be a household servant would seem a perfectly appropriate "education" for a poor girl, so by that standard Mrs. Deans is, in fact, "educating" her bound girl, and we are told she has had, over time, a string of these girls in her service.
They were also called "Home Children," because the British charitable organizations that sent them over were called "Homes." You recall, in Chapter 10, when Homer gives a ride to Myron, he speaks sarcastically about Mrs. Deans' care: "Mrs. Deans is what Ma calls a 'mother in Israel,' and no mistake. How many she's mothered! All these Home girls!"
A little known fact is that for Canadians with "Anglo" ancestry, if they've been in the country for several generations, there's statistically a good possibility that they descend from Home Children.
Here's an excellent short article:
And a Wikipedia article that puts it into a larger context. (They were sent to other countries as well. Britain officially apologized in 2010.)