Where children shouted 'Shell out!' instead of 'Trick or treat!'
The expression "shell out" in place of "trick or treat" evidently goes further back than even the 1940s, if we may trust the memory of Canadian poet Dorothy Livesay. Livesay was born in Winnipeg, Manitoba, but moved with her family to Toronto, Ontario, in 1920 when she was either ten or eleven, according to the Wikipedia article about her. In her poem "Halloweens" originally from Plainsongs (1971), but reprinted in The Self-Completing Tree (1999), she writes
I remember being twelve years old/wearing a black mask/over my eyes/a witch's cloak/over my shoulders/walking along Bloor Street with my sister/crying in the grocers' doorways/"Shell out! Shell out!/Halloween apples!"
This would have been in 1921, if Livesay remembers her age correctly. (Bloor Street is in Toronto.) The reference from Quill & Quire, volume 65 (1999), cited in Phil M Jones's answer, likewise appears to be specific to Toronto. Here is a lengthier extract from "We Want Candy: The Evolution of a Halloween Chant" [combined snippets]:
Every Halloween, I remember with a pang that when I was a child we went shellouting, and now the word doesn't exist, not even according to the Canadian Oxford Dictionary. "Shell out, shell out, the goblins are out!" we yelled. Some of my contemporaries remember witches, not goblins, but maybe that was a local variation — witches in downtown Toronto, goblins in the suburbanizing farmland to the west. And I have no doubt there were other local chants in other parts of the country. When I was seven years old my family moved to Winnipeg, and there the cry was "Halloween apples! Halloween apples!" Not very menacing, but they gave us candy, anyway (they knew we didn't want apples). Now everybody in the whole standardized, Americanized world goes "trick or treating." It's interesting that the spread of this American idiom should go through children, but it's not surprising — children always absorb, through their pores, it seems, the language that they hear around them.
The unidentified author here interestingly suggests that the sister's cry in Livesay's poem was an amalgam of Toronto and Winnipeg sayings. In any case the two calls cited here account only for Toronto (and its environs) and Winnipeg (hundreds of miles to the west of Toronto). An earlier discussion in "Once on Halloween...," in Grade School Teacher, volume 71 (1953) recognizes regional variations, too:
In Canada and the United States apples, pennies and candies are put into the youngsters' baskets when they call out either "Tricks or Treats," "Shell Out," or "Halloween Apples." What they call depends on the locality of their homes."
Margaret Atwood, who lived as a child in Ottawa, Sault Ste. Marie, and Toronto, as well as in rural northern Quebec, has this mention of "Shell out! Shell out!" in her novel Cat's Eye (1988) [combined snippets]:
On Halloween Grace wears an ordinary lady's dress, Carol a fairy outfit, Cordelia a clown suit. I wear a sheet, because that's what there is. We walk from door to door, our brown paper grocery bags filling with candy apples, popcorn balls, peanut brittle, chanting at each door: Shell out! Shell out! The witches are out! In the front windows, on the porches, the large orange heads of the pumpkins float, glowing, unbodied.
And Doug Taylor, Arse Over Teakettle: An Irreverent Story of Coming of Age During the 1940s in Toronto (2010) gives this account of Halloween hijinks in 1945:
Wednesday, 31 October 1945 was a moonless night, drizzling rain descending from a starless sky. ...
My brother and I ventured out around 6:00 PM Jimmy, Tim, and Shorty joined us. Children were already noisily scampering from door to door. When we arrived at the first house, we shouted the usual chorus: "Shell out! Shell out! We'll break your windows inside out!"
In some neighbourhoods, they chanted, "Shell out! Shell out! The witches are out!"
The words trick or treat were American and unknown to us. ...
Nicholas Rogers, Halloween: From Pagan Ritual to Party Night (2002) cites a mention of the chant in the Toronto Star during the 1950s:
The 1950s saw the taming of Halloween. "Shell out, shell out," remarked the Toronto Star quite lightheartedly, "for this is the week of treats or tricks. So light up the pumpkins and be prepared for everything from ghosts to colonial ladies, live bunnies, blackface comediennes and his satanic majesty."
Where children shouted 'Halloween apples!' instead of 'Shell out!'
Johanna, Graffiti on My Soul (2010) tells a historical anecdote that seems to be set in Regina, Saskatchewan (one province to the west of Manitoba) in 1946, in which she tries to gather candy on Halloween by calling "Halloween apples!"
Norah Lewis, ed., Freedom to Play: We Made Our Own Fun (2002), from the "Studies in Childhood and Family in Canada" series, focuses on "games activities, and amusements that were part of the culture of Canadian childhood from 1900 to the mid-1950s". Here is a description of Halloween in that era:
Bath Valentine's Day and Halloween were celebrated with classroom parties. The Halloween party gave youngsters an opportunity to wear masks, bob for apples, and play games. In urban areas, children usually went door to door demanding "Halloween apples" or "trick or treat," and children re expected to do a trick for their treat.
Dianne Linden, Shimmerdogs (2008) has a character from Edmonton, Alberta, (west of Saskatchewan) recall using the "Halloween apples!" expression:
Uncle Martin started telling me things. "You know, Mike," he said. "This makes me think of the first Halloween your mom and I spent in Edmonton. I was about your age. I dressed up like a Roman soldier. Your mom was a gypsy and we were upset because it was cold and we had to put snowsuits on over our costumes. We used to call out 'Halloween apples1' instead of 'Trick or treat!'"
And from Nora Findley, Jasper: a Backward Glance at People, Places and Progress (1992), there is this reminiscence of Halloween in the 1920s and 1930s in the Rocky Mountain town of Jasper, Alberta:
As near as I can recall in the 1920's and 30's, Halloween celebrations were restricted to children, but the festivities did not start until 6:00 p.m. It was not any part of the school program, except for the fact that the art lesson for the day was drawing pumpkins or witches. At precisely six o'clock, the children in some sort of camouflage, to hide their identity, called at all the homes in the neighborhood calling, "Apples, apples, Halloween apples!" Any other calls would have been considered rude. Old pillow slips had been saved by the mother to serve as appropriate bags in which to carry the apples. Very seldom did people vary from giving an apple to each and every child.
We as children, brought home our bruised and beaten apples, counted them to see who had the most, and turned them over to our mothers who spent the next few days making apple butter.
And Frank Howard, From Prison to Parliament (2003) has this recollection from Vancouver, British Columbia, in 1937:
I arrived at Mrs. Brown's home on Halloween. That night I was permitted to stand on the front porch with a lighted candle as the flame for a packet of firecrackers. I dutifully lit each one and tossed it out onto the front lawn. My head though was in Kimberley [in southeast British Columbia, not far from the Alberta border] where our Halloween dress-up game was to blacken our faces with a piece of burnt cork. Our cry at each house was not "Trick or treat"; it was "Halloween apples." Apples were available; Candy was in short supply.
The uncertain dividing line between 'Shell out!' and 'Halloween apples!'
Both "Shell out!" and "Halloween apples!" seem to be predominantly—and perhaps exclusively—Canadian expressions that children cry (or used to cry) on Halloween night. The literature for the Maritime provinces and Quebec (at least in Google Books) is silent on what children said there.
The expression "Shell out!" with various rhyming second lines is attested in Toronto and seems likely to have been used in other parts of Ontario as well. But the Prairie provinces—Manitoba, Saskatchewan, and Alberta—and British Columbia have multiple recorded instances of "Halloween apples!" and none of "Shell out!" Especially interesting is the tradition from the 1920s and 1930s in Jasper, Alberta, of collecting apples on Halloween for the mothers to make into apple butter.
It appears that "Shell out!" may have originated in Toronto; it was certainly in use there by the early 1920s. But somewhere between southeastern Ontario and southeastern Manitoba. "Shell out!" gave way to "Halloween apples!" which then prevailed across the western half of southern Canada.