Here's the index entry from Indiana University Sheet Music Collections. Who would have guessed how the first line was spelled?

Title: THREE LITTLE FISHIES
Title: Itty bitty poo
Composer: Dowell, Saxie
Name: Hal Kemp
Role: P
Publisher: Santly-Joy-Select Inc.
Place of publication: New York
Date of publication: 1939
Call Number: M1 .D48 Box: 086 Item: 081
Performance Medium: Piano, Voice and Chords
First Line: Down in de meddy in a itty bitty poo, fam fee itty fitty

A few more lines copied from http://shadowstar.ymakadomain.com/bards/unknown.htm:

Down in de meddy in a itty bitty poo,
Fam fee itty fitty and a mama fitty, foo.
"Fim," fed de mama fitty, "fim if oo tan,"
And dey fam and dey fam all over de dam.
Boop boop dit-tem dat-tem what-tem Chu!
Boop boop dit-tem dat-tem what-tem Chu!
Boop boop dit-tem dat-tem what-tem Chu!
And dey fam all over de dam.