I think one likely reason is that Sci-Fi is, mostly, also fantasy. It's about technology which produces results that are akin to magic, which inspires wonder. As adults, we can look at the current state of technology and science, and know that a person from more than 100 years ago would think we were using magic, but children are growing up with this as commonplace, and so get no sense of wonder out of it.
The two are tied. We can't see atoms, we imagine them based on scientific evidence. We can't see gravity, but we imagine it can be manipulated like any other physical phenomenon. Kids can understand acceleration, gravity, microgravity, but they imagine it can be turned on/off. I've always liked the scifi movies when something goes wrong and the artificial gravity shuts off. A small dose of reality. The line between real and fiction is real blury to children, but we adults have it figured out. Or do we. We have not given up on the search for space folding or tunneling to travel great distances in space, have we? To science affinity, and beyond!
You are comparing apples to oranges.
Kids aren't responding to science fiction, they are responding to science fiction stories. The concept of speculative fiction with hypothesized science or social changes isn't driving them. It is the stories, and examples of stories, and stories they create based on other stories, that entertains and inspires.
It isn't science, the investigation and understanding of the material and immaterial world about us that leaves them looking for a good story, it is the lack of science stories that leaves them dangling. We either tell stories of people involved in science or scientific research, or the business of organizing and conducting science. Stories of space flights, of ship handling and vehicle operation and maintenance are every bit as entertaining . . if the stories entertain the adults around them and their peers.
First, I think Brad and John is correct say it the 'stories' that appeal to kids and not the 'science' itself.
But, I think Ken brings up a more important point because once to strip away the stories, compared to science fiction, the truth is science is boring.
Now I know it may be heresy to say something like that but it is true. None of the stuff that makes science-fiction cool even remotely exists.
Iron Man suits, Light Sabers, Hoverboards, Flying cars, Jet Packs, Teleporters, FTL Space Travel, Bionics, VR, Bat-Belt Gadgets, Cloned Dinosaurs, etc. etc. etc.
Now, yes there are some stuff in the list above that sort of exists in various forms. But, none of it exists as we have imagined in stories, and even if it did it is not available to the average person.
Add to that the science kids are taught is just benign. I remember when we learned about electricity and batteries in seventh grade. We spent a week building a salt water battery for a electric toy car, only to have it charge overnight, and run for barely a minute in a little derby we held.
How is a little salt water batter car supposed to compete with a 5 story tall of Robotech Destroid? [and those are not even the Mecha that could fly in the series]
In the face of the realities of science and technology verses what Science Fiction can deliver, it is pretty obvious why Sci-Fi wins out.
I think the answer is pretty obvious. Scifi is designed to be entertaining whereas science is not. Science is generally pretty boring unless you absolutely love it and even then it can get pretty tedious. That is, actual science rather than science reporting (which is usually intended to be slightly entertaining) is a lot of hard work that is a lot of slogging. Scifi benefits from instant gratification that can be renewed by another flight of fancy at any time. This is exactly the kind of thing kids like. A protracted pedantic lecture on why things are the way they are is less exciting.
That being said, I don't think we need to apologize for science being difficult at times. Jazz it up, if you can, but you're trapped with the subject matter. You can't just make things up to make it more exciting.
As someone between childhood and fatherhood (I'm 23) I'd say that any child who's be appropriately exposed to true science and engineering will love both science and scifi, but favor scifi because its more fun. Let me explain.
My dad is a mechanical engineer for Nasa. When I was little, I got to see the F-117, SR-71, X-1, F-22 prototype, B-2... Dad even took us to 3 shuttle landings, and 1 shuttle launch. He was a rocket designer on Atlas, the liquid propellant engines for the Space Shuttle, part of the ICBM maintenance oversight and, most recently, Ares/Orion project engineer. So I grew up around the coolest aero/astronautics ever created.
He also did a great job showing me SciFi. The first 4 years of my life, we watched Spielbergs "E.T." at least once a month. Star Trek: TNG was also watched religiously, and my personal favorite, Back to the Future, is still in my collection at home.
I was taught the difference immediately. Dad explained how a capacitor can't have flux. Why you see astronaut space suits in ET. And that Dilithium is powerful, but difficult to work with. Having a geek for a dad, who knows the relationship between the true science and science fiction allowed me to grasp what the difference between dreams and reality truly are.
Now, onto your question, "Why do Kids prefer scifi..." Scifi is easy. Its consumable. You watch it, enjoy it, and let creativity run rampant. Anything you can dream, can be made reality in scifi.
Science isn't that easy. I'm an electrical engineer now, and having spent the better part of a decade in school studying calculus, physics, circuit theory, device physics and so on, I must say, its fracking hard. In the time I spent in school, hollywood came out with dozens of Scifi movies, TV too, but in that time the shuttle has been grounded, consumer electronics has become a fast paced cluster duck and engineers are never mentioned or consulted in gov't spending decisions. Heck, even Neil Armstrong pleaded with congress NOT to ground the shuttle. Science isn't fun. Its tough, getting the physics down is hard, getting funding is near impossible, and what comes out of it? Some teenager gets an iphone. whoop dee doo. Engineers/scientists... don't beat the bad guy at the end. We don't have midichlorians in our blood streams or drive Deloreans. (Some of us do drive deloreans, but there's no fluxing or fusion)
Fact is, science just isn't as cool as scifi. But a healthy understanding of the difference is critical, and kids can still love both, just prefer the entertaining one.
Science fiction, at least in the minds of the kids, is about transcending current limitations. It's the ultimate teen fantasy.
Real science is full of limitations. No faster than light motion, conservation laws, all those pesky laws of thermodynamics. Laws laws laws. It's the ultimate teen nightmare.