We wanna,
We wanna,
We wanna wee

If you don't stop for us,
We're gonna do it on the bus!

Rick tells us 'We Wanna Wee' has evolved and the version listed has been added to by successive generations. He says "It concludes:

after a while it'll trickle down the aisle
and you'll get your feet wet!

(ed: he said it had evolved and expanded, but sadly those lines were all he sent in)