Fines are too low to deter the organisers, who can take up to pounds 2,000 a day from stalls. In some areas, councils are unaware of the problem or have no resources to tackle it.
One flower boy died and another suffered head injuries in 1985 in separate road accidents, prompting Hertfordshire County Council to crack down on child employers. Under the Children and Young Persons Act 1933, it is illegal for under-16s to work more than two hours on a Sunday. Council by-laws also bar under- 18s from selling goods in the street. Offenders face maximum fines of pounds 400 for each offence but even if reconvicted, say the police, they are unlikely to face jail.
In 1985, Christopher Stephens, 14, died when he slipped and fell under the wheels of a car on the A405 near Watford, yards from the flower stall he helped to run, and Jamie Archibald, 12, suffered head injuries when hit by a car near his stall on the A1 at Hatfield. No action was taken against Christopher's employers but two men from a Watford florist's connected with the other accident admitted five offences. They were each fined pounds 155 with pounds 50 costs.
The same shop still employs children for up to 10 hours a day to sell flowers on main roads leading to the M25: up to 30 young people aged from 15 are believed to be involved in the weekend work. One, suspended from school, said he also worked on Friday afternoons. A girl who said she was 15 was working on a road from the M25 towards Epping, Essex. She was protected only by a hamburger van about 100 yards away and a phone box. If attacked, she said, she would ring her mother in Watford.
One mother accused the florists of preying on hard-up families. Her husband is on a low wage and she allowed her son, 15, to work. 'It gave him pocket money, but it is slave labour. They work from 7am, and sometimes don't come home until nine or 10 at night. The florists are exploiting them.'
The Watford children are paid pounds 20 to sit all day. Their only protection against bad weather is the polythene-covered wooden flower rack. In cold weather they are given portable gas heaters.
One boy of 15 told how he stopped working three weeks ago after the florist refused to pay him for two days' work. 'They said that I was light on the stock.' He claimed that children as young as 12 are employed.
The youngsters are recruited through word of mouth and advertisements in newsagents' windows. They meet at pick-up points on estates around Watford and are taken by lorry to 10 sites off the M25. None of those interviewed knew that they were working illegally or were in danger.
The Automobile Association says that one in eight accidents on trunk roads and motorways are caused by vehicles stopping on hard shoulders.
Records show children have been employed by florists and fruiterers in Hampshire, Oxfordshire and Surrey at least since 1977. Roadside selling is also common in Essex and Kent.
Action taken against the child employers varies considerably. Some, such as Hampshire, give three cautions before prosecuting. Essex gives a verbal warning. Surrey was unaware of any problem, now or in the past, but said it would take action if necessary. There is a history of children selling at Oxfordshire roadsides but there are no records of any prosecutions in the county in the past 14 years.
The Children's Society believes that tough new legislation is needed. Ian Sparks, the society's director, said: 'Operations of this type are exploiting young people's navety.'
The Independent on Sunday and Children in Focus have passed their files to the appropriate authorities.