David Walliams once told Simon Cowell his son "needs plastic surgery".
The 46-year-old comedian claims he made the quip about his fellow "Britain's Got Talent" judge's four-year-old boy Eric - who he shares with partner Lauren Silverman - after Simon had asked David to say who he thought was better looking out of their two kids.
David - who has five-year-old son Alfred with his ex-wife Lara Stone - said: "Simon Cowell is so competitive. He's competitive about every single thing you could be competitive about.
"He even said to me one day, "Who do you think is better looking - my son or your son?"
"I said, "Well, my son is obviously better looking, but the good thing is you're rich so you can pay for your son to have plastic surgery."
"He said, "Lauren, Lauren, David just said Eric needs plastic surgery." "
David and Simon have formed a bromance since appearing as judges on "BGT", and the former "Little Britain" star joked the pair love and hate each other "in equal measure".
He said: "There's nothing we wouldn't say to each other. We're like brothers. We sort of love each other but hate each other in equal measure.
"He once said to me, "You're only happy when I'm unhappy and I'm only happy when you're unhappy."
"It's really weird relationship, and very genuine."
David would love to star on an "X-rated version" of "Britain's Got Talent" alongside Simon, but joked such a show would probably "finish our careers".
Speaking on ITV's "Lorraine", he added: "We had a guy on ["BGT"] who happened to be a Mr Gay UK, and I said, "Thank you Mr Gay UK, we'll now go over to Mr Bi-Curious UK."
"The producers were hooting with laughter and I was like, "Can you use that?" And they said, "No, no, no."
"We really want to one day do an X-rated version of the show, where we can really say what we think.
"I think it would finish our careers for good."