"I said to Simon, 'Nothing can prepare you for how much you are going to love that child,'" David told The Mirror. "'You will love that child much more than you love yourself.' And he replied, 'That's not possible.'
"But I know Simon is great with kids. We have a lot on the show and he is superb. He is brilliant at talking to the kids and is very warm with them. I think he will be a great dad and he will really enjoy it."
Comedian David, 42, celebrated the birth of his own son Alfred with model wife Lara Stone, 30, last May.
"It is one of those things that when it happens you embrace it," said David. "You might think you will be this kind of dad or that kind of dad, but until it happens you don't know."
David Walliams and fellow judge Simon
David and Simon reunited in January as judges on the latest series of Britain's Got Talent, just weeks before Simon announced his comeback on The X Factor UK.
"I'm absolutely thrilled to be returning to The X Factor UK," Simon said at the time. "It's been three years since I last judged the competition and I'm excited to find a global superstar again."