A story which includes things that don't exist* = fantasy.
A story which details events that could reasonably, truly happen = not a fantasy.
* Ghosts, vampires, zombies, unicorns, etcetera.
Both BUFFY THE VAMPIRE SLAYER and WITHOUT A TRACE have dealt with helping people, having a dangerous job, dealing with interpersonal relationships, and coping with loss. However:
BUFFY THE VAMPIRE SLAYER: Fantasy.
WITHOUT A TRACE: Not a fantasy.
Both THE GOLDEN COMPASS and BECAUSE OF WINN-DIXIE boast determined young girls who keep their favorite animal (almost) always by her side and who have questions about her parents. However:
THE GOLDEN COMPASS: Fantasy.
BECAUSE OF WINN-DIXIE: Not a fantasy.
I could say THE GHOST WORE GRAY by Bruce Coville is about overcoming prejudice and THE DOLL IN THE GARDEN by Mary Downing Hahn is about consumption, but they still have ghosts in them and thereby drift into the land of fantasy. THE GHOST WORE GRAY and THE DOLL IN THE GARDEN are both good historical mysteries for kids that include friendly ghosts. If I had a nickel for every time I read either of these books . . .
Some books toe the line between genres or blend them into a delicious smoothie. (Mmm, smoothie. I'll have soy milk and strawberries in mine!) Is THE INVENTION OF HUGO CABRET by Brian Selznick a graphic novel? No, not in the way that collected comics or manga are, I suppose, but it's certainly not a picture book either. HUGO is an illustrated novel, with its pictures just as important to the storyline as the text.
I enjoy dystopic futuristic novels that detail advancements in technology, but I'm quick to point out which stories are just that - futuristic and plausible sci-fi - as opposed to outright fantasy. I really enjoy the UGLIES sequence by Scott Westerfeld. It doesn't have any unicorns in it, but it does have hoverboards, screens, and tracking devices which may be closer to being in existence than we think. After all, some of the spy gadgets I saw on COVER UP and other spy-based television programs twenty years ago are now being used by people on a daily basis. Consider webcams, for example. They are not unlike the little cameras or projection screens the good guys used during briefings. Think about all of the programs that are available on cellular phones and similar hand-held devices. Those are based in pixels and chips, so they aren't the same as unicorns.
Maybe someday Unico will gallop through the fields while talking to Stephen Colbert on a hands-free (hooves-free?) cellular earpiece. Until that day comes, if someone writes a fictional book about it, I would shelve it in fantasy.
Genre Study: Fantasy
Fantasy Novels for Kids and Teens
Funny Fantasies for Kids and Teens
Fairy Tales Retold