Revealing mistake: While Nick and his father are driving home with the giant baby behind (an obvious green screen projection), the baby breaks a tube from the car and throws it to the floor. Nick bends down to pick up a prop located offscreen, to add up for realism, but the problem is that he bends down before the baby throws it, so the whole trick is revealed.
Revealing mistake: When Mandy falls inside the Porsche, her stunt double is noticeable because she is way older than her.
Revealing mistake: When the family is running around trying to settle down the big Adam, Adam picks up Nick. From the back view, Nick is obviously a doll.
Revealing mistake: In Las Vegas, when the baby is about to pick the car and throw it to the floor, the close shot shows an obviously fake arm and hand.
Revealing mistake: After Adam passes the car by the palm tree, when he slides it and makes sparks, Nick's hands are not real but from an obvious miniature.
Revealing mistake: When Adam falls face-front in Vegas one can notice that Nick and Mandy have been replaced with two stiff toys.
Revealing mistake: After the giant baby grabs the giant bunny he then grabs his dad's head and the baby is replaced by an obvious inflatable doll.
Revealing mistake: When Mandy is hanging loose by the car's edge and Nick is grabbing her, there's a shot with a bluescreened background where Nick says "but you gotta help" where Mandy and the car are all blurry and faded and the image is doubled. Something went really wrong in that special effects shot.
Answer: She may have said goodbye to Adam but it just wasn't shown.