Children will often make up words to songs, because they don't understand the original words, and their words make more sense to them.
One sister sang (when she was little, of course!):
"Whole Wheat, Whole Wheat, Whole Wheat" to the tune of "Holy, Holy, Holy." She may not have known what "Holy" meant, but "Whole Wheat" was the flour that my dad was baking bread with!
Another sister had this rendition of "Low in the Grave":
"Low in the gravy lay". Just depends how you break the words apart!
I confess, I had a song that I didn't know the true words to until I was a teenager! We had a college tape with a song that included the words "Oh sinner go free". I thought it was "Oh synagogue free"...I wasn't sure what exactly that meant, but that's what I HEARD when I listened to that tape!
I'm sure that those of you with kids in your life have heard a song or two messed up. Do share please!