The 21-year-old star, who is a judge alongside the music mogul on 'The X Factor' USA, was shocked when she heard that Simon and Lauren Silverman were expecting a child together and she is angling to play a major role in the child's life.
Quizzed on whether she was surprised about the news on 'The Tonight Show with Jay Leno' on Tuesday, she laughed: "Who wasn't?"
She then added: "I don't know how it did, I just ... it's not my business to ask but it happened, basically though I did say if I'm not the not the Godmother I would be p***ed"
Leno then enquired: "Would you like to be the godmother?"
To which Demi replied: "Yeah!"
Demi was recently given a goat as a birthday present during a trip to Africa and she thinks an animal would be a great gift for Simon when he becomes a dad.
She said: "I should give him a goat, yes."
Demi has previously revealed fatherhood has melted Simon's cold heart making him less mean-spirited on 'The X Factor' judging panel this year.
She said: "He's [mostly] the same, but maybe he's gotten a little nicer this year."