Employers are legally obliged to register workers up to 16 with local education authorities, but in practice the majority ignore the system.
This means that there is no control over the hours children are working, or whether they are filling unsuitable jobs - such as on building sites or in factories.
The GMB general workers' union has signed up 200 children after visiting schools and offering pupils free membership in a deal that entitles them to free legal advice if they have problems at work.
The recruitment drive, launched in a pilot programme in Newcastle upon Tyne and due to be extended next year, was devised after research revealed that an estimated 600,000 children were working outside the law in evening and weekend jobs.
A report from the Labour Research Department last year found that seven out of eight children who worked were doing so illegally, while only 15 out of 108 education authorities had child-employment officers.
Last week, a study by Unicef, the United Nations children's organisation, suggested that one-quarter of British 11-year-olds were working - raising concerns over the effect on their education.
Chris Preston of the GMB's northern office said that the union wanted to offer them some form of protection. Those who are not registered are not covered by insurance at work, so they may not receive compensation if they have an accident.
Mr Preston said: "The by-laws dealing with children's employment are outdated and not designed to cope with the situation in Nineties."
The recruitment drive allowed the union to find out more about the extent of illegal employment of children, he said.
Mounting concern over the welfare of those employed outside the law comes as the Department of Health prepares to abolish the two-hour limit for children working on Sundays.
Draft plans would permit 13- and 14-year-olds to work for five hours and 15- and 16-year-olds to work for eight hours. At present, a 15-year- old can work for two hours on schools days and Sundays and eight hours on Saturdays. Under the changes the weekly permitted total of 20 hours would not change.
Local authorities have by-laws covering children's hours but in practice most follow the the department's guidelines.
Chris Pond, director of the Low Pay Unit which researches and campaigns on pay issues, said that many working children were at risk of accident or injury.
A survey of 2,000 children in Birmingham revealed that one-third had had some form of accident, including being cut by knives or stuck through the hand with needles.
"Excessive work not only exploits children but can have an impact on their education or achievement in terms of homework or attentiveness in class," Mr Pond said.
The GMB has also launched a campaign to sign up college students, who are increasingly working during term time as well as during vacations to supplement their grant.
A pilot scheme offering students at the London School of Economics membership for 10p a week is to be extended next year to other universities in the South-east, including Cambridge. The union offers students legal cover in the event of an accident or tribunal and advice and information on employment rights.