There is a lovely recording of this by Burl Ives. There are more verses, but this is enough for little ones; you can treat each verse as a separate song or put them together.
There’s a little, white duck sitting on the water,
A little white duck doing what he oughta.
He took a bite from a lily pad, flapped his wings
And he said: “I’m glad
I’m a little white duck sitting on the water.
Quack, quack, quack”.
There’s a little green frog swimming in the water,
A little green frog doing what he oughta.
He jumped right off of the lily pad
That the little duck bit
And he said: “I’m glad
I’m a little green frog swimming in the water
Glub, glub, glub.