'You know those men who turn 45 or 50 and just go to the corner shop for a bottle of milk and never come back?'

"He hit me," Karen tells me at last. "Yesterday Toby hit me."

"Karen! I don't believe it."

"Well, more of a smack, really," - strands of hair whip across her face in the freezing wind - "But I tell you, it hurt."

The children are far away, tearing ahead, towards the mound of grass where Peter Pan gazes eerily down from his fairy-encrusted pedestal. It's nearly four and the sky's molten, shot with liquid pink, air heavy with snow. Cigarette ends flash as people pass us, lights flicking on, Kensington Gardens fast emptying.

"Where?" I ask her. "Where did he hit you?"

"Here," Karen touches behind her ear, "I had to take Nurofen." She bites her lip, calls to Hetty, 18 months, who has let go of the pushchair and is running off, pink snow-suited arms waving. "He lost his temper. I said something he didn't like."

"What, for God's sake?"

"That he wanted to change the world, but couldn't be bothered to change a nappy."

"But I thought he was so good about - "

"He was, with Lucy, it was a novelty. Not any more - he wished we'd never had Hetty, you know."

"Oh, Karen."

"He doesn't like me," she says almost to herself. "Even the sight of me." Geese are standing in distant, tentative crowds on the frozen Serpentine.

Lucy (nine), Karen's eldest, Jacob and Chloe are already at the statue, suspiciously eyeing the boy who wouldn't grow up. Raphael, who was a baby in a pushchair last time we came, hoots with excitement.

"Look," Karen points. "A squirrel." The animal watches us from a few feet away, paws wrapped anxiously around a nut. Hetty is about to rush at it, but Karen grabs her hood.

"Why are they so tame here?" asks Jacob.

"They weigh up the danger against the possibility of getting fed," I say.

"And I know which one wins," Lucy says.

I've known Karen more than 10 years, since we sat in wine bars together and pulled the wings off each other's boyfriends. When she met Toby, he was working on a travel magazine, but gave it up to write full-time - she was a legal secretary and kept them both. "Actually, he never wanted kids, you know," she says - and I suppose I do know, but, like her, I've blanked it out - "too keen on doing his own thing."

Peter Pan listens smugly to this tale of domestic woe. There are crows and baby rabbits at his feet - and our own babies, worshipping with their little, grubby, mittened hands, waiting to be spirited off to fight Captain Hook. "Come on, Lost Boys, who wants to get a drink at the cafe?"

"Why are they called Lost?" asks Lucy, frowning.

"They fell out of their prams when they were babies." Karen roughly straps a rigid, struggling Hetty into the pushchair, clips the harness.

"Were they properly strapped in?" Chloe asks, concerned.

"Good question," I reply. Could the Child Accident Prevention Trust have prevented the flight to Never Never Land?

"He hates the weekends," says Karen. "Hates all the child care, being with the kids, the meals, the cleaning up. All day Saturday and Sunday, it's all you do isn't it? He'd like to shed his life and start again. You know those men who turn 45 or 50 and just go to the corner shop for a bottle of milk or a packet of fags and never come back?"

"Does Lucy know?" I ask her.

"Not that he hit me, but she hears us yelling at each other. He said sorry, but it didn't seem like much, just that weird, meaningless little word." Karen's crying now, "It's kind of the beginning of the end, isn't it?" she fumbles for a scrap of tissue, "Should I leave him to it, do you think? How do you leave someone you're so used to?"

I think of my mother who, at 33, bravely left my father when - but not just because - he hit her. I remember the cramps in my stomach when I lay in bed and heard them fighting, the almost physical relief when she left and took us with her, the various messes that followed. I say nothing.

We reach the little kiosk-cafe by the playground. It's far too cold for ice-cream, but Karen gets them for the kids anyway, weak cappuccinos in Styrofoam cups for us. I watch her smiling at the man in the kiosk and think how beautiful she is - long, strawberry-blond hair and a heart-shaped face, mouth smooth and curly like rose petals. I used to measure everything by her face.

"Careful, you guys - the cones are thin and crumbly." We get the travel pack of baby-wipes ready. The children stand in a chilly, serious-faced circle, licking.

It's snowing properly now, perfect, floaty, feathery flakes. "So what would you do?" Karen asks me and I suddenly remember all the times I haven't liked Toby all that much, but have ignored the feeling for her sake.

"Depends if you still love him," I say.

"No such bloody thing as love," says Karen quickly. "Not in that sense. You don't fall in and out of it - it's whether someone's pleased to see you in the morning, an on-going state of happiness."

"You don't love him," I inform her.

"I don't love him," she mouths after me, practising.

"A year from now you'll be somewhere else entirely," I tell her, noticing how easy it is to predict these things for other people. And I look up and see Lucy watching us, edgy, biting her thumbnail, eyes darting nervily around.