MichaelR suggested The Unicorn Song. Shel Silverstein has written some daft songs for sure, but I wouldn't pick that one.
What about:
Freakin' At The Freaker's Ball
A Boy Named Sue
Don't Give A Dose To The One That You Love
Cover Of The Rolling Stone
Goodnight Little Houseplant
I Got Stoned And I Missed It
I'm So Good That I Don't Have To Brag
Never Bite A Married Woman On The Thigh

Besides The Unicorn Song, he also wrote a bunch of other songs for kids:
Sarah Cynthia Sylvia Stout Would Not Take The Garbage Out
Someone Ate The Baby

The man was a genius. His song, Cover Of The Rolling Stone got Dr. Hook on the cover, but Shel never made it himself. His liner notes for Ramblin' Jack Elliott's first album are brilliant.