Amanda, who is back on the judging panel this year alongside Simon, David Walliams and Alesha Dixon, said Simon has been busy making plans for his unborn child's arrival.
"You can't prepare yourself like he thinks he can," says Amanda at the Britain's Got Talent London auditions. "He's got all these plans like where he's flying to and where he's going to take the baby, and they're not going to work.
"I just keep laughing at him. You know the amount of stuff you have to take around with you when you've got a baby?! it's just not going to happen."
Music mogul Simon, who is expecting a baby boy with girlfriend Lauren Silverman this month, has also showed Amanda the baby scan.
"I''ve seen a picture of the baby on a scan," Amanda continues. "It looks like Simon! He's got the same nose. It's crazy.
"Simon's not going to realise how much he's going to utterly love that child. That's the thing that's going to destroy him. You just love them so much."
Amanda, who is mum to daughters Holly and Lexi, has already taken her kids along to meet Simon at the auditions. "Holly came with me and sat on Uncle Simon's lap. He loved her. She snuggled into that big chest and she had a biscuit with him!
"He was so sweet and she was so chilled with him. She was like her mother flirting!"
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