Nonsense stacked on nonsense: a fiddling cat, a moon-jumping cow, a laughing dog, a dish that runs off with a spoon. None of it needs to mean anything, which is rather the point at this age, since the rhythm and the sheer daftness of the images are what land. Good for pointing at each character as it appears.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
Hey, diddle, diddle!
The cat and the fiddle,
The cow jumped over the moon;
The little dog laughed
To see such sport,
And the dish ran away with the spoon.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition