The first purpose-built professional theatre for children in Britain is about to be unveiled. Most guests at next week’s official opening of the pound;13 million Unicorn Theatre in London will have seen no more than architect’s drawings, but, for a group of 30 young people now in their first year at secondary school, there will be a sense of ownership. For three years, one class from Tower Bridge primary school, the nearest school to the Unicorn site just behind City Hall on the south bank of the Thames, have been “young consultants”.

In 1998, before the foundations had been dug, Tony Graham, the artistic director and a former teacher, decided to use the building project as a focus for sustained work with local children. This fitted into the Unicorn’s policy of holding in-depth workshops, already established when the company lost its base at the Arts Theatre in central London in 1999 and became peripatetic.

The theatre’s first “consultants” were a Year 6 class from Tower Bridge who, by the summer of 2001, had looked at venues along the South Bank, including the National Theatre and the Globe. Alison Barry, the Unicorn’s education and youth director, says: “They tracked the historical trail of theatre locally and examined the need for a purpose-designed theatre for children.”

In the autumn, the Year 6s handed over to a younger group of Year 4s who were to act as long-term advisers on the project. “They have been to the site several times and it’s fabulous how they still see it as their own theatre and want to bring their families,” says Tony Graham. “But to begin with, their response was, ‘What’s a theatre?’.”

Jane Shallice, described by Mr Graham as a retired teacher “who leads from the front and shoots from the hip”, was appointed to lead the pupil consultancy project. Her first task was to answer that basic question: what is a theatre? The children loved the National Theatre building, seeing it as a huge playground. At the Royal Court they were fascinated by the blood-red wall made of lead to insulate the auditorium from traffic noise.

Asked if they would change the seating, one suggested different sizes for different ages, while another wanted “more cushiony” material. Dominique told Jane Shallice that the foyer was rather dim and small. She says she knew that Ms Shallice was listening “because her expression stayed serious”.

Even more important than the shape of a theatre, however, is its purpose.

The young consultants saw a number of Unicorn productions during the three years. Ms Shallice says: “Even when we went to Clockwork (a Unicorn opera adapted from Philip Pullman’s book) at the Royal Opera House, so many of them were drawn into something that was very difficult and grown-up.” The children themselves say that the world of theatre is no longer mysterious.

As Bobby puts it: “It’s for people like us.”

Tony Graham visited Tower Bridge school to lead workshops, and the children discussed theatre “as a place to provoke, to instruct and to entertain”.

Then they began to make their own plays. The theatre’s architect, Keith Williams, also visited the school a number of times: “My role was to try to explain what an architect does, and they were excited to be included. For me it was an opportunity to be reminded how kids think, how much they know and how quick they can be to grasp complex things.”

Soon they were thinking about dimensions, proportions and scale. Their suggestions went, says Mr Williams, “from the practical to the outlandish”.

The most extreme was that the floors should be made of chocolate. “I said I didn’t think we’d get that past the fire codes.” Someone suggested that the seats should be sofas, which led to a discussion about distance from the stage and sightlines.

Tony Graham wants the theatre to have a roughness about it, “not to be pristine, pure, ‘don’t touch me’, but uplifting at the same time”. All the rooms have natural light, which is unusual in dressing rooms and rehearsal spaces. The practical needs of catering for children have not been ignored either; stair rails and wash basins have been installed at different heights, for instance.

There are two auditoriums, the larger of which, the Weston, is clad in copper and described by Williams as a “theatre in the sky”, approached by a staircase. In the shape of an amphitheatre, it draws on Greek tradition and acknowledges the importance of storytelling. There is also a smaller studio theatre called the Clore and an education space which will open with an installation, In the Box, designed by children aged six to 11, featuring soundscapes, poetry, painting and sculptures expressing their hopes, fears and stories.

The contribution of the Tower Bridge children has largely been to influence thinking and underwrite adult decisions. But their role was more obviously important in one particular area. With the help of artist Bhajan Hunjan they learned about public art, that it could be many things, made from all kinds of materials and integrated into floors and ceilings. They designed and decorated the hoardings to go around the building site with Hunjan and were, says their teacher Neil Crawford, delighted when their work appeared in the local press. Then, in teams, the children took part in a competition to design a piece of art.

The winning team was invited to help choose the professional artist whose work would be incorporated into the design. They sat on a panel with the architect and representatives of the theatre, asking questions of the shortlisted artists and taking part in the discussion afterwards. Keith Williams wanted artwork that was “integrated into the building, not a clip-on. Something that should enhance the architecture.” The result, agreed by all, is two pieces of artwork in the foyer featuring illuminated abstract computer images.

When Caryl Jenner, pioneer of children’s theatre, named her company in 1967, she said “the Unicorn only exists because you believe in it”. This new Unicorn has plenty of believers, some of whom have had what Neil Crawford describes as “indelible life experiences” in bringing it to life.

For further information go to www.unicorntheatre.com. Tickets: 08700 534 534. General enquiries: 08700 535 500