"Of course not," she says. "Holly's four, for God's sake. It was Tallulah, who's eight. She often comes round. She's a perfectly nice girl, but I had to give her a bit of a talking to. It's a really tricky one - I just don't know whether to tell her mother."
"But what is there to tell?" I ask, laughing.
Isabel fiddles with a slice of lemon, "I don't think it's all that funny. Tallulah may be sexually aware but Holly and Tom are very young."
"But she can't know what she's doing at eight? Surely it was just a bit of fun?"
"Fun?" Isobel stares at me as if I'd just suggested a double junior subscription to the Playboy Channel. She bites her lip. I hear the disapproving fizz of her water.
"I mean," I struggle to qualify, "a game. It's not at all sexual at that age - or, OK, if it is, surely only in a perfectly normal, natural, experimental way?"
"Natural?" bays Isobel, picking at her watercress garnish. "But Holly was licking his ..."
"But only licking!" I exclaim hopefully. "Like a lollipop. Children are always licking things, for heaven's sake. I agree you should discourage it, but it's hardly deep-throat fellatio. I think you're making rather a big deal of it, that's all."
Isobel looks upset. "I'm sorry, but I think it is a big deal," she says, ever so quietly.
We order cappuccinos. "I'm really surprised you're so laid back," she says. "Wouldn't it bother you at all? A friend of a friend had something similar happen and went straight to the headteacher."
I laugh at the idea of dragging in the poor headmistress. "About something that happened at home?"
"Yes. Why not? It was someone else's child. What could she do?"
"But what was the headteacher supposed to do about it? Who was it anyway? Anyone I know?"
But Isobel smiles and shakes her head and flicks a Hermeseta into her coffee. "I'm naming no names. Well, she complained to the parents, of course."
"Really?" I say, more coolly now. "How repulsive."
"Oh? I don't think so," says Isobel, narrowing her calm, brown eyes at me. "Someone's got to take responsibility.
"For what exactly?"
She gives me a chilly stare. "For behaviour that can lead to ... other things."
It occurs to me that I haven't known Isobel long and maybe we just don't like each other very much. Or is this simply the next hurdle - arguing with our friends over how we tell our children about sex? As we divide our bill, I wonder whether Isobel will think twice in future before sending Tom and Holly round to play in our casual bordello of a house.
My mother introduced my sisters and me to sex in the most open, logical and gentle way I can imagine. I've copied her methods with my own children. Good sex education is when kids can't remember when they first heard about it - it just feels like they've always known.
When I was two, I knew I came out of Mummy's tummy. By three, I'd grasped by precisely which exit, and by four or five, I knew how the seed that turned into me had got in there. By the time I was old enough to demand precise details, the idea of penises going in and out of vaginas was so absolutely everyday as to seem dull. Which is just how it should be until you get older and then it turns weird and embarrassing all over again. I don't remember licking any little boys' penises, but then how would I know?
"Nothing you could think or imagine, nothing you could want to know about sex could shock me," our mother boldly assured us right from the start. "You can be pretty certain I've seen and heard it all." Who cares whether or not it was true? It was what we needed to hear - a beacon of comfort in the thickening gloom of puberty. "This is routine," you could reassure yourself as you sprouted, bled, fantasised, feared, in the Charlie-scented privacy of your own room. "Everyone's done this. I'm normal."
And I want the same easy comfort for my children. So penises are cheerfully compared in the bath, nipples routinely discussed over supper. "Mummy," says Jacob later that afternoon, as he stabs a pasta shape with his fork. "At school today there was a bit of a situation."
"Oh yes? What happened?"
"Well, Lester came into the loo and he pulled down his pants and ran around pointing his penis at everyone."
"Well," I grate Parmesan on to pasta. "You know what to do when that happens, don't you? He was only doing it to get attention, so the best thing is just to ignore it and look away, or to look really bored. If you react and make a big deal of it, that's exactly what he wants and he'll do it again."
"Penis, penis, peanut," chants Chloe, dressed in nothing but a green dayglo tutu, silver tiara and pearls - a four-year-old Cicciolina with milk moustache.
"But he just got it out and ran around holding it," persists Jacob, head on hand. He sighs enthusiastically at the mere memory.
"So did you just ignore it?"
"Well, so what did you do?"
"We all screamed and ran away."