I really wish I had time to write this story myself.
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Poo and Pee Polly is a doll you give lemonade and chocolate pudding. After a little while, they come out the appropriate orifices.
The inventor makes a life size one, and uses it for sex and scat / WS fetish.
What happens when he's out of town and his teen son finds it?
Or when his daughter finds the male version?