"What do snowmen eat for…

A popular winter joke for children is:

Q: What do snowmen eat for breakfast? / Q: What is Frosty the Snowman’s favorite cereal?
A> Frosted Flakes.

The joke has been cited in print since at least 1998.

Wikipedia: Frosted Flakes
Kellogg’s Frosted Flakes or Frosties is a breakfast cereal, produced by the Kellogg Company and consisting of sugar-coated corn flakes. It was introduced in the United States in 1951, as Sugar Frosted Flakes. The word “sugar” was dropped from the name in 1983.

Generic versions, such as store brands, are also available. Unlike many cereals (Cheerios and Rice Krispies, for example), Frosted Flakes shares its name with generic competitors.

Google Groups: microsoft.public.inetexplorer.ie4.outlookexpress.stationery
It’s Beginning to look alot like Christmas!
Katrinka
12/2/98
(...)
What is Frosty the Snowman’s favorite cereal? Frosted Flakes!

Google Groups: alt.teens.penpals
A Question I have about sex.
merry mcgrath
5/29/99
Laugh Lines: 1. Why couldn’t the teddy bear eat? Because he was stuffed. 2. What’s a snowman’s favorite cereal? Frosted Flakes.

Google Groups: alt.abuse.recovery
Not much of a Joke
Silverleaf
12/12/01
(...)
> But can you tell me what SnowPeople (snowmen and snowomen and
> snowpeople) eat for breakfast?
Frosted Flakes!!

Google Books
October 2008, Boys’ Life, “Think & Grin,” pg. 63, col. 1:
Justin: What’s a snowman’s favorite cereal?
Jacob: What.
Justin: Frosted Flakes!
Justin Brundage, Medical Lake, Wash.

Google Books
March 2009, Boys’ Life, “Think & Grin,” pg 56, col. 2:
Luke: What does a snowman eat for breakfast?
John: Beats me.
Luke: Frosted flakes!
Luke Collins, Somers, N.Y.

Google Books
The Big Book of Laugh-Out-Loud Jokes for Kids
By Rob Elliott
Grand Rapids, MI Revell
2013
Pg. ?:
Q: What does a snowman eat for breakfast?
A: Frosted Flakes.

Twitter
Ashley Marie
‏@ashleyymohrr
What do snowmen eat for breakfast? Frosted Flakes
11:43 PM - 2 Dec 2015

Twitter
Mike Wabe
‏@MikeWabeCrier
Watch out for my Christmas “crackers”. Here’s one to start with. What do snowmen eat for breakfast? A. Frosted Flakes #Christmas
8:31 AM - 6 Dec 2015

“What do snowmen eat for breakfast?”/“Frosted Flakes.”