While it is true that children are able to use their imaginations in ways that adults cannot, adults use imagination to create the next great thing. If we use some synonyms for imagination, such as dreaming or vision, most of us would agree that adults are better at this--they are the more mature forms of imagination.
Children are generally thought to be the most capable of imaginative play. One reason is that they haven't really used many of the devices we use everyday. I look at the dishwasher and I see a chore. My son looks at it and see a jungle gym or a garage to park his cars in. Since he doesn't use the dishwasher, he doesn't have a preconceived notion of that object. Children are far more capable of turning an object into something else or pretending that an object exists when it isn't there. As we grow and learn, we lose some of this ability.
You may be trying to compare apples and oranges. Children are certainly less inhibited in their idea-creation process, so their imaginations can take off into realms of fantasy. On the other hand, adults have the experience and background awareness to be able to piggyback off one idea and imagine connections and ideas related to a topic or a previous idea in ways that would not be possible for children.
I think the ability to imagine things that are unreal is probably about equal between each demographic - children, adults, men and women.
The distinguishing factor may be the ability to suspend our sense of reality and to engage in fantasy/imagination. In this way, children are more apt to engage in fantasy and adults are more likely to daydream and keep their imaginings in their heads, I think.
I think children because as the previous poster mentioned they don't impose limits on what they can imagine and they aren't constrained by any reality since they have not been exposed to as much in life.
Children have very few informed opinions. Rather they use their imagination to create the world as it should be; their creative thinking knows no bounds, and the wondrous thing, especially in smaller children, is that they are willing to share their vision and imaginative wanderings unabashedly. Children don't worry if their idea is good enough or not, they just dream for the sake of dreaming. Few adults have that lack of reserve.
I think it is adults because we have more experiences to use to help us imagine things. However, there are adults who lose the ability to be imaginative because they are too concerned with what is likely to happen. Instead of being creative and imaginative, they get too bogged down in real life and cannot imagine things that are improbable.
I think children = adults