Old MacDonald Had a Farm is a traditional children song, that's been occasionally featured in The Simpsons. It was sung by Grampa in "Homer's Barbershop Quartet". A parody was sung by the children of Springfield Elementary School in "The Mook, the Chef, the Wife and Her Homer". The song would also be whistled by Maggie Simpson in "Whistler's Father".
Lyrics
Homer's Barbershop Quartet
Grampa: Old MacDonald had a farm.
E I E I O.
And on this farm, he had a chick.
The swingin'est chick I know.
With a wiggle wiggle here
and a wiggle wiggle there.
Whistler's Father
Maggie/Homer: (Whistles the tune of "E I E I O")
Lenny (Speaking): Now do it backwards.
Maggie/Homer: (Whistles the tune of "O I E I E")
Lenny: Now sideways.
Homer: Hey, come on, she's just a baby.
Moe: Who is?
Homer: My talent.
Moe: Makes sense.