Dennis Kelly is pondering. Sitting backstage at the Cambridge Theatre in London's West End, as a packed matinee crowd pours out of the building, I've pointed out that people who like musicals love Matilda – but so do people who don't. 'Yeah, I've noticed that as well,' says Kelly, who co-wrote the show with comedian Tim Minchin. 'I think it's because Matilda is a bit irreverent - a bit naughty.' It's an apt description for a show that not only contains a song called 'Naughty', but actively champions deviant behaviour (for the right reasons).

Now, after over seven years in London, Matilda is finally touring the UK. This year's dates include stops at the Curve in Leicester, the Birmingham Hippodrome and Manchester's Palace Theatre. And just announced for 2019 are new dates at the Edinburgh Playhouse, Theatre Royal Plymouth, Bradford's Alhambra Theatre, Bristol Hippodrome, Southampton Mayflower and Norwich Theatre Royal.

Like the Roald Dahl book that inspired it, Matilda The Musical is the tale of a fiercely intelligent young girl, trapped in a family of limited intellect. Painfully aware that nobody else is going to change her life for her, she finds her own path to happiness. Commissioned by the Royal Shakespeare Company in 2010 to adapt Dahl's well-loved book for the stage, Kelly behaved in much the same way as the show's heroine – and did it his way.

'I think you have to make it your own,' he says. 'Because Dahl wrote books – and he was brilliant at it – but he didn't write Matilda as a play. If he had, he wouldn't have done it like that, it's a totally different structure. So one of the big jobs for me was teasing the story into a different shape. Because if you're too respectful, then you end up just doing exactly the same as Dahl did, and things which work brilliantly in the book just wouldn't feel right on stage.'

With the script in place, including numerous holes for songs to slot into, Kelly was introduced to Minchin – and everything changed. 'That's when I realised I had to do something more than just leave holes – I had to actually leave space for the songs to do stuff,' he recalls. 'But Tim was amazing – we had tussles, because in any musical the text and songs are always competing for space, but because Tim is such a laugh, it was always good natured.'

One thing both men had in common, was a lack of knowledge about the unwritten rules of musicals and how they're structured. 'There are a number of things you're absolutely supposed to have at certain times in a musical,' says Kelly. 'Like an "I want" song at one point or a duet at a certain moment. And I think Matthew Warchus the director knew about these rules, but he was clever enough not to tell us. I certainly didn't know them and wasn't interested in them, and Tim was the same.'

The result is a musical that, like Matilda herself, does things a little bit differently. Darkly funny songs like 'Miracle', that subtly lambasts parents who over-praise their children; the moving 'When I Grow Up' performed on swings that swoop out into the audience; and the remarkable 'Revolting Children', a rebel song in the classroom that brings tears to the eyes with its sheer brilliance. Kelly and Minchin may have broken the rules, but they were well worth breaking.

'We just wanted to make something that feels good,' says Kelly. 'and something that we ourselves would want to watch.' In the end, they made something an awful lot of people want to watch. Eight years in the West End so far, a spell on Broadway followed by tours of the USA and Australia have put Matilda The Musical in front of thousands of audience members, who echo the approval of critics and award bearers (the show has won almost 80 at the last count, including seven Oliviers and four Tonys).

credit: Manuel Harlan
Kelly's clever script, Minchin's witty lyrics, Warchus' astute direction, Rob Howell's striking set design, Peter Darling's dynamic choreography, the superb adult cast and all the other wonderful components of Matilda The Musical notwithstanding, it's the children who really send the crowds away smiling. The young actors who play Matilda and her classmates are always selected and nurtured with care and attention – although they almost weren't a part of it.

'We tried lots of things at the start and did a lot of experimenting,' explains Kelly. 'As I was writing it, I thought all the characters were going to be played by adults – and at one point we even toyed with the idea of Matilda being played by a puppet. Our worry about children was that Matilda's story might be too hard for a kid to do. But then we ran a workshop and it was fantastic. It's funny – now we look at the show and think of course, it has to be played by children – but back then, we just didn't know.'

When it came to who they wanted to watch the show, however, Kelly and Minchin had no such confusion. 'We were quite clear right from the start that we wanted everyone to enjoy it – kids and adults,' says Kelly. 'But we wanted adults to enjoy it as kids, in a way. Because we've all been kids, we know what it's like and we can all access that really easily.'

Matilda the Musical is touring the UK in 2018 & 2019. Find out how to get tickets.