“Trick or treat” is a Halloween phrase that began in the 1920s. A popular rhyming extension has been cited in print since at least 1948:
 
“Trick or treat, smell my feet, give me something good to eat.”
 
Another verse—“If you don’t, I don’t care/ I’ll pull down your underwear”—has been cited in print since at least 1988. A Canadian verse—“Not too big, not too small, just the size of Montreal”—has been cited in print since at least 1995.
 
   
Newspapers.com
24 October 1948, Deseret News (Salt Lake City, UT), “Trick or Treat: Halloween Will be Here Very Soon,” pg. N-15, col. 2:
Trick or treat! Trick or treat!
Please give us something good to eat!
 
Newspapers.com
28 October 1957, Fort Lauderdale (FL) Daily News, “Pint Sized Goblins Take Over Thursday,” pg. 3-B, col. 1:
“Trick or treat, trick or treat, give us something good to eat!” they’ll cry in tremulous tones.
 
Newspapers.com
25 October 1963, Oshkosh (WI) Daily Northwestern, “UNICEF Aids World’s Children” by Garry Cleveland Myers, pt. 2, pg. 6, col. 2:
“Trick or treat” used to mean “Give me something good to eat or I’ll play an ugly trick on you.”
 
Newspapers.com
2 November 1964, Press-Telegram (Long Beach, CA),  “George Robeson’s Nightcap,” pg. B11, col. 4:
Their pitch was a poem, an obnoxious one as small boy’s poems often are:
 
“Trick or treat, smell my feet, give me something good to eat.”
 
Newspapers.com
2 November 1965, El Paso (TX) Herald-Post, “Editor’s Corner” by R. W. Lee, pg. B2, col. 1:
‘Trick or treat, smell my feet,
“Give me something good to eat.”
 
Newspapers.com
30 October 1966, Chicago (IL) Tribune, “Trick or Treat—Kid’s Delight,” sec. 10, pg. 5, col. 3:
... say that among their antic tomorrow night they’ll shout, “Trick or treat, Smell my feet. Give me something good to eat.”
   
Google Books
Birthdays, Holidays, and Other Disasters
By Thomas James Mullen
Nashville, TN: Abingdon Press
1971
Pg. 36:
Armed with great sacks, they work both sides of the street with the traditional ultimatum, “Trick or treat, smell my feet, give me something good to eat.”
 
31 October 1971, Sunday Register-Star (Rockford, IL),  “It’s time for goblins” by Cathy Cryer, pg. C1, col. 2:
“Treat or treat, smell my feet; gimme some’in’ good to eat.”
 
Google News Archive
26 Octobe 1978, Pittsburgh (PA) Post-Gazette, “Area trick-or-treaters fear little, they say” by Anne Arden, PG North, pg. 3, col. 1:
“Lots of my friends,” he said, “say, ‘Trick or treat smell my feet give me something good to eat.’ But I don’t say it ‘cause it’s bad. Neither does my brother.”
 
Google Books
My Mother Is Not Married to My Father
By Jean Davies Okimoto
New York, NY: Putnam
1979
Pg. 86:
The door opened and Harry yelled at this lady, “Trick or Treat. Smell my feet. Give me something good to eat.”
   
Google Books
American Children’s Folklore
By Simon J. Bronner
Little Rock, AR: August House
1988
Pg. 80:
69. Trick or treat
Smell my feet
Give me something good to eat
If you don’t I don’t care
I’ll pull down your underwear.
   
Newspapers.com
31 October 1990, Missoulian (Missoula, MT),“Tricks and treats” by Mea Andrews,  pg. 1, col. 1:
“Trick or treat, smell my feet, give me something good to eat. If you don’t, I don’t care, I’ll pull down your underwear.”
 
2 November 1995, Winnipeg (Manitoba) Free Press, “A night of unbridled greed” by Tom Oleson, pg. A16, col. 5:
In the last few years, the favorite variation, at least in my neighborhood, was: “Trick or treat, smell my feet, give me something good to eat; not too big, not too small, just the size of Montreal.”
   
Google Groups: alt.drunken.bastards
Jane & Chris
2/24/97
(...)
Trick or Treat,
Smell mt feet,
Give me something good to eat!
 
Not too big, not too small,
Just the size of Montreal!
     
OCLC WorldCat record
Trick or treat, smell my feet, give me something good to eat
Publisher: Hollywood, CA : Zombi a Go-Go/DGC, 1998.
Edition/Format:   Music CD : CD audio : English
 
OCLC WorldCat record
Trick or treat, smell my feet
Author: Diane De Groat; Jason Harris; Peter Pamela Rose
Publisher: [Pine Plains, N.Y.] : Live Oak Media, 2008, ©1998.
Edition/Format:   Audiobook on CD : CD audio : Fiction : Juvenile audience : Primary school Book : English
Summary:
Gilbert is excited about the costume he is planning to wear in the Halloween parade at school, until he discovers that lots of others have the same costume.
 
Uproxx
Jimmy Fallon Sang ‘Trick Or Treat, Smell My Feet’ As Bob Dylan
WRITTEN BY JOSH KURP
11.01.12
Did you know there’s more to the Halloween ultimatum classic “Trick or Treat, Smell My Feet” than its title, followed by, “Give me something good to eat/If you don’t, I don’t care/I’ll pull down your underwear?” I did not (I also just realized the song could send you to jail, what with its message of, “Give me candy, or show your goodies”), but according to this not-at-all poorly designed website, there are alternates versions, including:
 
Trick or treat, trick or treat,
Give me something nice to eat,
If you don’t we don’t care,
We’ll put money in your underwear