Texas has many heat jokes. “I wish it would rain—not so much for me, because I’ve seen it, but for my seven-year-old” is an old joke that dates from at least the 1940s.
   
     
Google Books
I Give You Texas: 500 Jokes of the Lone Star State
by Boyce House
San Antonio, TX: The Naylor Co.
1943
Kessinger Publishing
2004 (reprint)
Pg. 6:
An inhabitant of far West Texas said sadly:
“I wish it would rain—not for my sake because I’ve seen it rain but on account of my seven-year old boy.”
 
“Guide to Flooncy”
A Guide To Flooncy
from the book “How To Be A Texan- No Experience Necessary” (out of print, alas),
by Sam Huddleston
(...)
You hear a Texan saying: “I wish it would rain - not so much for me, cuz I’ve seen it, but for my 7-year old.”
 
Arizona Heat Jokes
A sad Arizonan once prayed, “I wish it would rain - not so much for me, cuz I’ve seen it—but for my 7-year-old.”
 
Google Books
Pastor Tim Cleanlaugh Collection
by Tim Davis
Victoria, Canada: Trafford Publishing
2001
Pg. 139:
A sad Texan once prayed, “I wish it would rain—not so much for me, cuz I’ve seen it—but for my 7-year-old.
 
Google Groups: alt.hahaha
From:  m. mckeller
Date:  Wed, Jul 29 1998 12:00 am
 
A sad Texan once prayed, “I wish it would rain - not so much for me, I’ve seen it—but for my seven year old.”