Are there limits to what can be depicted on stage? If there are, Swiss theatre director Milo Rau seems hell-bent on testing them. Recently described as 'the most exciting artist in Europe', Rau trained as a sociologist before founding theatre and film company The International Institute of Political Murder in 2007. His speciality is rigorously researched reconstructions of socio-political conflicts, from the Congolese civil war to Putin's show trials. But Five Easy Pieces, which has its UK premiere in March as part of Manchester's SICK! Festival, represents controversial territory even for Rau. A collaboration with Flemish children's theatre company CAMPO, it uses seven child actors to tell the story of infamous Belgian paedophile and serial killer Marc Dutroux. As Rau heads to the UK for the first time in 15 years, he tells us about the moral and aesthetic implications of working with children – and why Five Easy Pieces has nothing to do with breaking taboos but everything to do with the violence of theatre.

Do people expect the founder of the International Institute of Political Murder to be less smiley?
For some people it's strange that I'm quite normal and simpatico. They are expecting a king of darkness. But I am not. This piece is a funny play, even if it's a dark subject. Like all my projects it has a lot of jokes and fun and poetic moments in it.

How has training as a sociologist influenced your theatre?
For me, going to the places behind a play and meeting the people involved is very important. Even if I was making a Chekhov play, I would go to see where Chekhov lived and try to find people living in the situation described. For Five Easy Pieces, we tried to meet everybody involved in the Marc Dutroux case: Dutroux's father, the investigating policeman, the families of the dead and the surviving children. This is really important: for political theatre to involve people who were involved.

Was it always clear to you that theatre should be like this?
From the 19th century up to the end of the wars, theatre became a very closed thing for a small part of the bourgeoisie. It had to open up, or it would have lost the whole territory to TV, film and journalism. I think we are living now in a new renaissance of European theatre. People are trying new ways of telling things. We have toured in 30 different countries, but not in England. We were never invited! This could be the last play I have in England. But I hope it is the start of something.

Five Easy Pieces is your first collaboration with children. What took you so long?
Well, you know, I'm really interested in Greek tragedies and art house movies and folk music – things that children don't really like! CAMPO always ask directors who they are sure have no idea about working with children. I don't know why they do this. I think I was the most absurd choice they could make. Then I made a very simple calculation: I had Belgium, I had children, so certainly we had to do a play about Dutroux. But in fact, this Dutroux story is really just an allegory to think about emotions, to think about mimicry, to think about what happens to us when we see children playing adults.

Has anyone told you you shouldn't be doing this?
No, no. We had two journalists involved from the start so we wouldn't have a scandal, so nobody could 'find it out'. Several of my projects were crushed for that reason. So I decided to throw open the doors from the beginning.

You borrow the title from Stravinsky's piano exercises for children. What are the children learning in your play?
The first lesson is mimicry, how to play old and sick. The second is about biographical design: you are playing a policeman who is finding the murdered children, how do you construct his character on stage? The third is called ‘essay on submission', and is about the relationship between actor and director. The fourth is about emotion: the children have to play parents who have lost their children, they have to cry on stage. Then comes the last lesson, rebellion: how to revolt against everything they have just been asked to do.

How much did the seven child actors already know about Marc Dutroux?
A lot. In Belgium he's an iconic figure. Every child knows more or less what he did. But it's more like in a fairytale, and they also played it like this. It's like if I'm asking you to play Richard III. You will not think, Oh fuck, I will be traumatised, he's such a monster. When the children understood that we could meet the father of Dutroux – who lives more or less in front of the theatre where we rehearsed – it was quite funny and strange for them.

You worked with two advisers and a child psychologist. Were you alert to a danger of traumatising the child actors?
I was careful to make a clear professional frame: offstage you can play, but when you are on stage I'm talking to you as an actor and not as a child. Also we had a long casting, to see who is really strong enough to do this. With professional actors we improvise a lot. But children really have to know where is the stage and where is the reality. So everything, even if it seems to be improvised, is scripted. It's also very Brechtian. You can see how the play is constructed. In special moments you can see the children looking to the audience to see, "Is it working?" They are playing victims, but in a way we are their victims. And the play is also about this.

You've said that the play is deliberately drawing a link between paedophilia and the act of making theatre with children for adults. Can you expand?
I always thought it was strange that, when you see children's theatre, you can feel there is an adult director. But he's not on stage. That is why we have my assistant director, Peter [Syenaeve], on stage as my alter ego, the performance coach. You can see him leading the children more and more into a dark space of paedophilic play. For example, he asks one of the young girl actors to get naked to play a scene. He says, ‘but you did it in the rehearsal, why don't you do it now? You have to do it, that's how Dutroux did it'. This is the reality of theatre, this is what theatre directors do. But when you do it with children, you understand the violence of direction. I wanted the play to reflect this: that the strategies of a director are the strategies of a criminal."

Was your task more problematic because this is a play made by children for adults, rather than for children?
I would prefer if all the children could see it. It is forbidden for children under 14, and in some countries under 18, to see this play. But of course a lot of brothers and sisters of the actors have come to see it. And they liked it a lot, they understood it totally. Here you can see this overprotection, how we treat children in Europe. Every film on YouTube is more traumatising than Five Easy Pieces!

What have you learnt about children from making this piece?
When you are working with children, tragedy and comedy become one. As adults we feel we have to make the choice between good and bad, fun and tragic. We shouldn't. When you see this play, and are one minute crying and the next laughing, you understand. Audiences say, ‘it is so hard to look at this play, but in the end you feel free'.

Talking of which, you've been described as one of the ‘freest minds of our time'. Has making Five Easy Pieces tested even your liberal limits?
I was never a very authoritarian director. I never said ‘make this' or ‘do that'. But when you are working with seven children, you can't just be friends. That for me is a new situation: them and me. For my liberal mind, sometimes to say to a child: ‘Shut up now, we have to restart, you didn't do it well'… that was a test!

What have you learnt about theatre from making this piece?
The so-called postdramatic movement has led to playing a character being seen as childish: you shouldn't act, you must always be very ironic. Children have a way of being very theatrical and totally themselves. I re-learnt the strength and the magic of theatre: it is real and not real at the very same time.

You've just premiered your next piece, 120 Days of Sodom, a collaboration with actors with disabilities inspired by the Marquis du Sade. Are you ever concerned you'll run out of taboos?
Oh no, no, no. There are a lot of taboos! But also, it is not only about taboos. In Five Easy Pieces there is nothing that is even trying to break a taboo. It's more in the minds of people watching it.

Do children help theatre to uncover adult taboos?
Yes, that is absolutely what they do. I was shocked, when we first interviewed children about Dutroux, how much they knew about DNA tests and everything. These children know more than I know about criminal science! And with my daughters, who are nearly seven and ten, I discover how much they know anyway – and how much we think they shouldn't know. This is the internet age. In the end, even in the lives of our children, all taboos are broken.

Milo Rau's Five Easy Pieces is premiering in the UK as part of SICK! Festival 2017 at HOME, Manchester on 25 March.