There are restrictions on the hours of work for children, defined in employment law as below the compulsory school age, nominally 16.
Effectively, children may only be employed after reaching the age of 14 and then only on light work. There are general restrictions limiting work to between 7am and 7pm, but not during school hours. Work carried out outside school hours on schooldays and on Sundays is limited to 2 hours.
Local Authorities have powers to vary these hours and conditions, so approach your local authority (eg City or Borough Council) for details of the rules in your area.
24 Feb 2012: When do you start getting paid?
The Interns: Getting treated like a dogsbody?