I originally saw this rhyme at SurLaLune Storytime, then adapted it for the flannel board using clip art. I used this elephant and this bathtub.

One elephant in the bathtub going for a swim.
Knock, Knock, (Clap twice)
Splash, Splash, (Slap knees twice)
Come on in! (Motion with both hands to come in)

Two elephants...

Three elephants...

Four elephants...

Five elephants elephants in the bathtub
Going for a swim.
Knock, Knock,
Splash, Splash,
They all fell in!