Parents are facing a testing week. South Park: Bigger, Longer, Uncut, the ultimate in crude, animated adult entertainment, is coming to a screen near your home and whether or not you have ever heard of Cartman, Stan, Kyle and the frequently killed Kenny, you can be sure that your son or daughter has.

The foul-mouthed film, which is the first feature-length version of the hit American television cartoon, has been rated a 15 in Britain, but younger children are planning ways to foil the age restriction.

Teenagers who sneak in to see the film will be treated to a barrage of profanities which makes the 'kick ass' talk of the Channel 4 series sound tame and the intimate and explicit details of a homosexual relationship between Saddam Hussein and Satan.

As the film went out on general release on Friday, the multiplex managers up and down the country were instructing staff to ask young patrons for proof of identity, preferably with a photograph.

'So far we have not had too much trouble today, but we are expecting a lot more over the weekend,' said Vinay Dave, the manager of the Warner Village cinema in Leicester Square, London.

But he added that out-of-town multiplexes on sites with other attractions such as swimming pools and bowling alleys had more problems with children trying to defy age restrictions on popular movies.

Tell-tale signs that your child has sneaked in to see the South Park film might include increasing use of foul language and the tendency to break into song about the sexual proclivities of a nameless uncle.

Ironically, the plot of the film revolves around the efforts of Cartman, Stan, Kyle and Kenny to gain entry to an obscene comedy cartoon from Canada. Eventually they pay a vagrant to pretend he is a parent in order to get in. The makers, Trey Parker and Matt Stone, who weave their libertarian message into the lyrics of the movie's big musical production numbers, are mainly mocking the outrage of conservative middle America, as epitomised by Kyle's mother.

She actually starts a war with Canada because she is so incensed by bad language that her son has picked up from the movie. 'Violence is all right as long as nobody says anything dirty,' she exhorts the troops.

Outside the cinema in Leicester Square on Friday night, 12-year-old Daniel from Israel strongly disagreed. He had managed to persuade his parents to see South Park with him, only to be turned away at the door because of his age.

'It is not fair,' he complained. 'It may have bad words in it but is only a funny film.' Mark Muncy, a Presbyterian minister from New Jersey with an 11-year-old son, was pleased to see tighter restrictions on the movie here than in the US, where it has an R rating, which means young teenagers can see the film with written parental permission.

'They make these films about children and they can't go and see them,' said his wife, Fran. 'It creates conflict in the family.'

Paul Samuels, a father of two from Hertfordshire, said he thought the most important thing was for children to understand what swearing was. 'I tell my children that it doesn't matter so much about not saying "fuck", as long as it is only used for really important things. It makes you look stupid if you use it all the time.'

In the film, Cartman eventually has a V-chip implanted in his head which gives him an electric shock when he swears.

But Chris Steinhart, a pilot from Greenwich, said his children seem to have their own natural V-chip, as they dislike him swearing in front of them.

'I would not like my kids to feel comfortable about swearing,' he added. 'It is the way of these cartoons to make children feel comfortable with being obnoxious.'

This ignores the key factor in South Park 's success: that the incongruity of obscenities spilling from the mouths of children can be very funny.

But perhaps 'that movie has warped my fragile little mind', as Cartman says.