Earliest example of a song that subverts the expectation of a bawdy rhyme (better explained in OP)

Cafe Society
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Any kid who listened to more than one episode of Dr. Demento knew “Shaving Cream”.

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This is the theme song to "My Black Son, a sit com Peter Griffin imagines on Family Guy.

My black son, my black son
Now each day my heart is growing bigger,
Don’t even remember sleeping with that lady but I did…