It’s certainly an eclectic genre, as is demonstrated by the breadth of the programme for next month’s “Movies for Kids’’ season at the National Film Theatre. This takes in animation (Astrid Lindgren’s Pippi Longstocking), silent classics (Buster Keaton in Sherlock Junior, from 1924), subtitled films (the Swedish film Ronja the Robber’s Daughter, also from a story by Astrid Lindgren), and swashbuckle rs (a new print of The Adventures of Robin Hood, from 1938). There are films about animals, extraterrestrials, cyborgs - and even children.

For nanny-statists there is an obvious answer to the question: a film is suitable for children if there is a letter “U’’ on the posters. The U (universal) classification is the only one that has remained unchanged since the British Board of Film Control was set up in 1913 in a climate of concern about juvenile delinquency. It did not, however, mean that a film had been deemed ideal for children, merely that it would not do them harm; a blank screen would have merited a U certificate.

Indeed, some moral crusaders would undoubtedly have preferred it that way.Margery Fox, of the Headmistress’ Conference, giving evidence to an inquiry in 1917, was asked whether “the very impressiveness of the cinema’’ aggravated the danger of cinema-going to children. She replied: “Certainly. The better it is, the worse it is’’ - not the best basis on which to create a quality children’s cinema.

Unhealthy excitement remained a cause for anxiety, as did the danger of fire and other accidents occurring when children are crowded together in confined spaces.

By the 1930s, however, the spread of Saturday morning matines was an acknowledgement that cinema-going had become a legitimate pastime. The major distributors made these screenings the focus for clubs with a variety of wholesome activities and a Boy Scout ethos, including own songs,loyalty oaths, circulars, competitions and outings. There was the ABC Minors, the Odeon Mickey Mouse Club, the Gaumont British Junior Club, and the Odeon National Cinema Club. The matine programmes, with their mixture of adventure serials (Flash Gordon), comedy shorts, cartoons, documentaries and Westerns, did much to define the category of “children’s film’’.

Most of this material had not, however, been made with a child audience in mind. It was drawn from the “B’’ section of normal cinema programmes, and the cinema clubs were an opportunity to show matine audiences enticing (but appropriate) extracts from the evening features. What defined the matines as “children’s cinema’’ was the age of the audience, not what was on the screen.

The idea that films could be produced specificall y for children took root with the emergence, in the 1940s, of J Arthur Rank as the dominant force in the British film industry. Rank, a flour miller, had become interested in cinema because he was perturbed by the shortage of films suitable for showing to Methodist Sunday schools. In 1943, he set up a children’s film department (later Children’s Entertainment Films) at the Rank Organisation, under Mary Field, a sociologist who shared Rank’s preoccupation with raising standards of taste.

In 1950, the leading distributors in Britain set up the Children’s Film Foundation with finance from the government. Rank was appointed chairman and Mary Field executive officer. The CFF made its own films - which tended to involve a lot of piracy, smuggling and petty theft, with young heroes and heroines foiling the villains - and it imported and dubbed foreign films. By the mid-1950s, it was making four features a year, plus two shorts, a serial and three magazine programmes. The foundation was determined to produce only films of the highest quality (for example, using only original music), but it operated under considerable constraints.

Cost was the main inhibiting factor, but there were also problems with employing child actors or depicting stunts that children might try to copy,and with storylines containing any sort of love interest. The CFF’s productions - well received at film festivals - included The Boy Who Turned Yellow (1972), the last collaboration between Michael Powell and Emeric Pressburger. The CFF continued until the early 1980s, when it became the Children’s Film and Television Foundation.

Through all this change, there has been one constant: we haven’t got much closer to agreeing what constitutes a children’s film.

Terry Staples, joint programmer of the NFT season and author of the standard work on British children’s cinema (All Pals Together), suggests that the primary criterion is “a central character, usually a child, with whom children can identify”. This includes films such as Babe and ET, whose characters are not human, as well as The Railway Children and Pippi Longstocking, which have all-human casts. However, Staples’s list excludes Anastasia or Honey, I Shrunk the Kids (as Staples argues, the very title is a remark addressed from one adult to another, over the heads of their diminished offspring).

The idea that the defining element is a character with whom a child can identify implies a concept of childhood as a metaphysical, as well as a biological, state. Children can see themselves in Babe and ET because these creatures are small, powerless, lonely and under threat from a grown-up world they do not understand.

But powerlessness can also imply freedom from responsibility and other constraints of the real world. Pippi Longstocking possesses superhuman strength, lives by herself (her father is somewhere in the South Seas) and goes to school only when she feels a need for holidays (you can’t have holidays unless you have school, silly). Tintin, the boy reporter, is gloriously unencumbered with parents or other ties, apart from his dog, Snowy. He never has to meet a deadline - indeed, he never seems to file any copy at all - and yet manages to travel around the world without ever getting a rude letter from the bank manager. So they are only children, after all.

Unlike children’s literature, children’s films will always be an ill-defined, marginal species, unless they are made exclusively for video or TV. Big-screen movies cost a fortune to make, so producers prefer “family entertainment”, with a range of characters and some grown-up jokes thrown in to amuse the parents. But there will always be a need for films that adopt a child’s viewpoint on the world; it often makes more sense than the mature version.

The Chewits Movies for Kids season at the National Film Theatre starts on Saturday, August 1, and runs daily until Sunday, August 9, when there will be a special screening of the film which came top of a recent poll organised by the ‘Daily Telegraph’. (The poll results will be announced on the day. ) The season then continues through the month on Saturdays and Sundays. For bookings, telephone the NFT box office on 0171 928 3232.

‘All Pals Together, The Story of Children’s Cinema’, is published by Edinburgh University Press, 1997.

Terry Staples is currently compiling

a catalogue of children’s films, to be published this autumn by the BFI