Twilight, Catherine Hardwicke, 122 mins, 12AThe Tale of Despereaux, Sam Fell & Rob Stevenhagen, 93 mins, U

Girl meets boy, boy turns out to be a vegetarian vampire

There are two films based on best-selling American children's novels out this week.

The one with the loudest hype is Twilight, adapted from a "Girl Meets Boy, Boy Turns Out to Be Vampire" swoonathon by Stephenie Meyer, the latest "new J K Rowling". It stars Kristen Stewart as a 17-year-old who moves from her mother's house in Florida to live with her father in a rainy mountain town. She immediately notices she's being glowered at by Robert Pattinson (Cedric Diggory in the Harry Potter films), the school's strangely pallid dreamboat. He's not the ideal boyfriend, what with his blood-drinking, but he doesn't have fangs or turn into a bat, and instead of disintegrating in sunlight, his skin just goes a bit glittery. More importantly, he belongs to a coven of "vegetarian" vamps who stick to animal blood. That's all Stewart needs to know before she declares her unconditional love. But then some other vampires stride moodily into town, and they have no compunction about biting any necks that take their fancy.

It would be easy to play the story for knowing laughs, as Buffy the Vampire Slayer did, but the director grounds the supernatural hokum with some earthy naturalism: Stewart is authentically gawky, and the town has the misty dampness of a Lake District bank holiday. In other words, Twilight takes the story as seriously as it's possible to take a romance about two lovers with a 100-year age gap. There's no nudging or winking to break the spell.

There are flaws, though. We're given no clue as to why Stewart should be the only girl in decades to set Pattinson's unbeating heart aflutter. It seems her single personality trait is her willingness to snog a centenarian.

But maybe that's part of Twilight's appeal: its heroine is so blank that viewers can project themselves on to her. Between Twilight, Sex and the City, and Mamma Mia!, three generations of women are having their guiltiest romantic fantasies catered for at cinemas this year.

Their young progeny don't fare so well with The Tale of Despereaux, a dreary cartoon fairy tale about a stout-hearted, Dumbo-eared mouse (voiced by Matthew Broderick). At least, that's what some of the film's about. But for the first 20 minutes we see neither hide nor hair of the title character, which will prompt a lot of children to tug at their parents' sleeves and ask what happened to the mouse in the poster.

Before we're introduced to Despereaux, we first have to meet a rat (voiced by Dustin Hoffman) who ruins a royal banquet by falling into some soup prepared by a French chef. (Note to producers: in future, try not to remind viewers of an infinitely superior Pixar cartoon.) The queen is so appalled that she drops dead, and the rat plummets into a rodents' underground city which is all too similar to the one in Flushed Away. (Note to producers: in future, try not to remind viewers of an infinitely superior Aardman cartoon.) Oddly, one of Despereaux's two directors, Sam Fell, also co-directed Flushed Away, so he's essentially plagiarising himself.

There's a blip of humour when the eponymous daredevil mouse finally arrives, but even then he keeps having to make way for endless, gratingly whimsical digressions and flashbacks. It's left to a narrator to pull the storylines together, but she prefers to deliver pious lectures about honour and forgiveness, which only makes matters worse.

"The Tale of Despereaux, a Princess, a Serving Wench, a Refugee from Ratatouille, a Moralistic Narrator and a Genie Made Out of Vegetables" might not have been a very catchy title, but it would have been more accurate.