And yet I find all of those responses rude. Why can't anyone just address the situation head-on? I realize with adults such responses can be funny and stress-relieving, but with children I find those responses to be a flippant dismissal of the child's emotions.
I guess it's a "know your audience thing". Sometimes you need to say something out of ordinary to make them realise that they are dismissing your statement that what they are asking is not going to happen/not possible.
I suppose it's also a metaphorical snap of your fingers to wake them up, since lines like that are normally used in these types of situations:
Son: Gosh, I really really would like to borrow your car for *fancy event*
Mom: I'm not comfortable with that
Son: But it matches my date's purse! You need to lend it to me
Mom: No, really, I don't think that's going to happen
Son: But I really want to borrow it!
(repeat a few more times)
Mom: And I would really like to snog Nathan Fillion, but that's not going to happen, is it?