The reason for it is perfectly understandable. These plaster casts of some of Europe’s great architectural and sculptural marvels dating from the 18th and 19th centuries are certainly works of great craft, if not art themselves, and because of the ravages of 20th century traffic fumes Roman scholars come here to study this version of Trajan’s Column because the original is so badly eroded.

Over the years, however, schoolchildren have been unable to resist snapping off salient keepsakes from the brittle forms, which has meant a continuing repair programme of Forth Bridge proportions. But the notice is a metaphor for an attitude which has prevailed in varying degree at the VA for almost a century-and-a-half, that this is a museum for adults into which children enter under sufferance.

While the VA’s South Kensington neighbours, the Natural History and Science Museums, have been able to cash in on juvenile enthusiasm for dinosaurs or space technology, the VA has seemed content that it has little to offer children of pre-GCSE age.

The philosophy, however, is already different in the minds of the new generation of curators such as Gwynne Miles, the head of new projects at the museum. “We have to get away from the idea that the VA is in any way exclusive of children, and I have no lower limit in my mind” she said. “Children are perfectly capable of appreciating museum objects if they know why. We see the new Boilerhouse Building as the mark of our wholly inclusive policy, and we have learned from the other museums and believe we have as much, of a different sort, to offer younger children as they do” The proposed new building is already famous. It is, according to press reports, a black vortex of tumbling blocks rising over Exhibition Road and casting a shapeless shadow over the venerable Victorian and Edwardian wings of the VA on either side of it.

Comments have been predictable. “Outrageous,” said one critic, “Horrific, ” said another, “A tawdry affront to a national treasure,” charged a third.

Daniel Libeskind, the 50-year-old Polish American architect who conceived the building which he describes as “the key to the city that is the VA” is unfazed by them. He has been quickly labelled as a deconstructivist architect, and a post-modernist, none of which he believes remotely apply to him or his extraordinary building.

The image we have is, in fact, quite false. The model used in the May press launch was a working one made of dark grey plastic resin giving a menacing organic appearance. In fact, as a new model shows, it is to be a polychrome structure, the colours of the tiles cladding it having been scrupulously researched to take their shades from the surrounding buildings and from the decorations of the existing VA galleries.

Far from being sinister, the form and colour have been devised to appeal especially to children and their families, said Libeskind and his wife and business partner, Nina. But nor is it a crude primary colour representation of tumbling building blocks. “As I first entered the museum after I was asked to compete for the commission, I saw two words carved high on either side of the entrance, ‘Inspiration’ and ‘Knowledge’, and they have been my guide through the whole process.” He studied the career of the building of the VA from its inception in the 1850s to the 1980s restorations of its colour schemes.

But the grand Cromwell Road portals are not the most welcoming for young people who have little concept of what they will find within. He needed to address that as a priority.

“It was an absolute part of the brief that this was to be a building for children,” he said. “This building must orientate itself away from the idea that there is one kind of visitor only. They must all find that this museum is a home, that nothing is taboo to them. So we have a double storey lobby which has in it not only a restaurant and a bookshop, but a children’s play area, a drop point where adults can leave their children in what is going to be a kind of kids’ museum,” he said. The children’s area has been expanded from the earlier brief, said Gwynne Miles, and will now be a kind of hands-on art museum. Exhibits will change regularly, but there will be craftsmen at work on the applied arts of which the VA is the world’s leading repository: carving, modelling, pot-throwing, furniture-making even glass-blowing are going to be demonstrated.

There will also be areas for school parties to conduct their own activities, devised by the VA’s education department in collaboration with teachers. There will be two entrances in Exhibition Road, one for individuals and small groups and one for larger parties so that there need be no serious queueing.

Children will be given opportunities to try the crafts themselves. Sculpture will be on show with an encouragement to touch and feel. As well as the 21st Century Gallery and temporary exhibition space, the Pounds 42 million Boilerhouse, due to open in 2001, will house the new education centre, but original plans to put the education offices in it have been reconsidered. “It’s going to be such a public building with such an emphasis on access and participation,” said Miles, “that we felt as little of it as possible should be closed off, and we now think the offices should go elsewhere.”

In the Libeskind scheme, the Aston Webb Screen - the row of columns designed in the 1890s by the VA’s last architect, Sir Aston Webb, which have formed the Exhibition Road entrance to the museum since 1983 - will be removed to the other side of the new building to become the wall of a new garden devised to add another dimension for children, an outdoor gallery. High-speed lifts to the top floor, 43 metres and four storeys high, will give views over the museum’s existing buildings to the north and south, its award-winning Pirelli Garden to the east, and the other South Kensington institutions to the west. Visitors will then descend through the central core of the building on escalators, able to see everything on the floors they pass through.

“It brings people into the centre of the building and they can see straight away that the mysteries of it are not as bewildering as they might have thought,” Libeskind said. “When this building is completed,” Nina Libeskind added, “it will become the heritage for our children. We’re not thinking about our generation, we’re thinking about the generation who are going to be adults in 25 years’ time with children of their own. They will think of this as their building.”

Libeskind refers to his design as an “Ariadne’s Thread” which will make the VA’s enormous collections easier to comprehend. “The building is not one kind of thing; there are diversities, not only of the kind of people coming here, but of the kind of exhibit that will go in here - demonstrations, participation in arts and crafts of a very different kind than was available in the 19th century,” he said.

“We no longer believe, as we did then, that galleries have to be rather dark places where you speak softly. No-one, especially children, will feel they have no place here.” The structure itself is a wonder of new technology, devised by Libeskind in partnership with the design engineers Ove Arup: the strangely geometric tiles - and much of the decoration of the Victorian building is tiling - which he calls factiles, none of which seem to have the same form but all of which fit together perfectly, are not merely decoration but part of the structure of the building, so that no interior pillars are needed to hold it up and at the same time obscure views.

Not only did the building have the unanimous approval of the VA’s board of trustees, but the staff, who have been kept briefed, have also been enthusiastic for it, said Gwynne Miles. They have been aware, she said, that while the museum has always fulfilled a role with higher and further education students which has been renewed in recent years through joint courses with the nearby Royal College of Art, it has never had a rapport with younger children.

“We are not merely making a statement about that with this building, we are fulfilling something,” she said.

Libeskind himself, for whom this is a third museum project, repudiates the pigeon-holing of his building into any architectural school. “I don’t make buildings like that. I make them to fulfil a purpose, but I make them for the people that will use them. I make no apology for thinking about what children want to see in it: they will be using it now and for all their lives.”