Francesca Simon is irritated. It may be more than four decades ago now, but it still rankles that she was denied a batmitzvah. After all, she was a Jewish girl living in California - why did she have to miss out?

"It made me very angry," the author, most famous for creating the best-selling Horrid Henry series, admits, from the calm surroundings of her publishers in London. "We lived in Malibu and neither of my parents wanted to drive me to the synagogue. That was the reason. They drove my younger brother, of course, for his barmitzvah, but not me."

The 61-year-old does not appear to be the kind of person to bear grudges. But this was important then, and clearly still matters now.

"I liked studying and I wanted to go to Hebrew school. I didn't see why I shouldn't," Simon continues. "But my dad was writing [he was a screenwriter] and it would have been a hassle. It wasn't high on their priorities. I do feel very resentful about it."

Despite the resentment, Simon, is a delight to interview, being both friendly and enthusiastic. It's a pleasure to talk to someone willing to answer all questions and be honest, whether it is about the batmitzvah fiasco or her refusal to see the 2011 Horrid Henry film (reading the script was enough for her).

And it is Henry - an extremely naughty boy who never thinks of the consequences of his actions - who made the author's name. She first began writing when her son, Josh, now 26 and a theatre director, was just three, and had her first Horrid Henry book published in 1994. In fact, she stopped writing new Henry stories only last year (she thought, reasonably enough, that 96 - four for each book - were enough). Each book took her a year to write.

Henry is always in trouble; he's the opposite of his rather wet, but well-behaved younger brother, Peter, and children (mine included) have long enjoyed being able to relate to him, without actually having to do anything naughty in real life.

One key to the books, as opposed to many other children's stories, is that Henry himself never changes. He doesn't always get his come-uppance or suddenly become "good"; he just carries on behaving badly.

Some parents, however, blame the books for their children's poor behaviour, or won't allow them to read them "just in case". Simon is extremely dismissive of such people, and nails them incisively.

"I think it's foolish," she says simply. "A lot of parents are very anxious about their children not liking each other. There's a lot of denial going on, because they think that reflects badly on them.

"I think saying their children can't read my books says more about the parents than the children, and I'd like to know what they're finding disturbing, because what does Henry really do?"

Passionate about defending her most famous creation, Simon continues: "Show me a child who doesn't call their siblings names, hit them or pinch them. Horrid Henry and Perfect Peter are two sides of everybody and what Henry does is allow children a really safe place to channel those emotions. You get the thrill of being naughty and no punishment. It's like heaven."

She adds: "The message parents reading this with their kids are sending - and I think it's an important one - is 'I know sometimes you hate your siblings, I know sometimes you hate me, but you know what, I can cope with it, the world isn't going to fall apart. You have these huge emotions, but we can handle this as a family.'"

Henry, whose exploits have been translated into 29 languages, is not Jewish, but a careful reader might find a few Jewish influences in the series. These include Horrid Henry's Thank You Letter, which was inspired by Simon's son's barmitzvah.

"We spent a summer trying to make Josh write thank-you letters," Simon says, with a smile. "He could probably have written thank-you letters for every bar and batmitzvah child in the UK by the time we finished arguing about doing it. That's why I wrote that book."

There's also one Jewish sentence, although you may not have noticed it. It's in Horrid Henry's Car Journey, where the children (like so many) begin arguing about which side of the car to sit on.

"The dad says 'We haven't even started and already you're fighting,'" Simon says. "That's my one Jewish thing that's consciously in there."

Simon's newest book contains a character who is definitely not Jewish, and something of a departure for the author. Not only is Hel, the main character in The Monstrous Child, female, but to Simon's astonishment, she arrived without bidding.

"I was just sitting on the subway early one morning in New York and suddenly this voice came to me with the first sentence. And I wrote it down and though that's Hel, that's the goddess of the dead, talking about her brothers.

"I loved writing in the first person and it was very exciting to sit down and let this voice fill me. I never felt so much like I was channelling someone."

Simon, who left America when she came to study at Oxford, and now lives in Tufnell Park, is clearly delighted with her new book, which is aimed at older audiences of 12+. Hel, who is based on the Norse mythology which Simon has always loved, is half-human, half-corpse, wickedly dark, funny and clever.

There are giants and gods galore, and Hel makes an immediate impression. It is clear that Simon relates to her fictionalised queen of the underworld, and if you look carefully, there are shades of Horrid Henry in her too.

"I'm drawn to characters who don't care what other people think about them, who lack self-consciousness and don't care about being nice," says Simon. "Hel doesn't care if people like her or not. She says exactly what she wants."

She "probably is" a bit of me, the author adds, explaining: "I do like her. I find her very funny. I admire her tenacity and philosophy and her humour.

"I've made her like a teenage girl, who's filled with loathing and disgust and it was really interesting tapping into those feelings for her.

"There's also that sense of being hopelessly in love with someone who's never going to feel like that about you. Living those emotions made me quite sad."

And then Simon is reminded of life back in Malibu.

"It's not the same, but I grew up in California, and the aesthetic there was tall, slim, with straight blonde hair, and I was none of those things. And also I grew up in the '60s and '70s and the look for women was Twiggy and you're thinking 'this is the ideal and I sure am not it'."

Hel's story is the third in Simon's mortal gods series and appears to have become her favourite. But she still retains a sense of surprise that she is able to do a job she enjoys, and which was entirely unexpected.

"It wasn't an ambition of mine to write for children. If I hadn't had Josh, I'd never have done it, I'm 100 per cent sure of that," she says. "Having young children is boring, so I used to read to Josh a lot. I started reading to him at four months."

Simon is passionate about literacy, which brings her on to the issue of libraries. She seems almost in shock about the closure of so many libraries and campaigns against this regularly.

"I genuinely don't understand it," she says, looking pained. "I think it's barbarous. They're such an important community resource.

"Libraries are vital for children and vital for older people - they're the two heaviest users," she adds. "So closing them is essentially cursing the future generation. And in a country where one in three families don't have books at home, too. It feels like a bad dream and makes no sense."

Her campaigning will continue, as will her writing. And I'm guessing that there are other stroppy, strong-minded characters just waiting to be brought to life.

'The Monstrous Child' by Francesca Simon is published by Faber & Faber