At what point does the imaginary world of children?

People are in such a hurry for their children to "grow up" and lose that childlike innocence and wonder of the world. Why? Imagination should be encouraged instead of squashed. I don't mean encourage them to the point that they cant' function in the "real world", but there's nothing wrong with having an imagination and that should never be treated as something that needs to be left behind. If more grown-ups used their imaginations the world would be a better place, wouldn't it?