A Christmas riddle is:
   
Q: What Christmas carol is a favorite of parents?
A: Silent Night.
 
That is, parents prefer silence. The joke has been cited in print since at least 2001.
 
     
Wikipedia: Silent Night
“Silent Night” (German: Stille Nacht, heilige Nacht) is a popular Christmas carol, composed in 1818 by Franz Xaver Gruber to lyrics by Joseph Mohr in the small town of Oberndorf bei Salzburg, Austria. It was declared an intangible cultural heritage by UNESCO in 2011. The song has been recorded by a large number of singers from every music genre. The version sung by Bing Crosby is the third best-selling single of all-time.
   
23 December 2001, Yakima (WA) Herald-Republic, “Sarah Jenkins Column—Quiz, Quips Catch Humor of Holidays,” pg. E9:
“What Christmas carol is a favorite of parents?”
“Silent Night.”
   
Google Groups: alt.christnet.christianlife
What do elves learn in school?
Vera Six
12/18/03
(...)
What Christmas Carol is a favorite of parents?
?Silent Night
 
Twitter
Lights of Christmas‏
@WarmBeachLights
What Christmas Carol is a favorite of parents? Silent night.
6:06 PM - 20 Oct 2009
 
Twitter
Goldbook‏
@goldbook_ca
What is the favourite Christmas Carol of parents? ...Silent Night (har har har)
3:51 PM - 8 Dec 2009
     
My Town Tutors
Parent Jokes: Top 50 Christmas Jokes for Parents
Posted on December 1, 2013 by Paula Fontaine (Westminster, MA)
(...)
39. What Christmas carol is a favorite of parents?… Silent Night.
 
Google Books
Best Christmas Jokes For Kids 2
By Peter Crumpton
PeteyRF Creative
2015
Pg. ?:
What Christmas Carol is a favorite of parents?
Silent Night.
 
Google Books
Horrid Henry’s Jumbo Joke Book (3-in-1)
By Francesca Simon
London, UK: Orion Children’s Books
2015
Pg. ?:
What Christmas carol do Horrid Henry’s parents like?
Silent night.