School-age children can take part-time jobs from the age of 13. However, they can only do so with a permit issued by the local eduction authority. (This can be withdrawn if the child's school work appears to be suffering or if the authority have any other concerns about the employment).
There are also restrictions about the types of employment a young person can be involved with, the number of hours which can be worked and the earliest and latest times of the day for working.
See here for details:
http://www.direct.gov.uk/Parents/FamilyIssuesA ndTheLaw/ChildrensRights/ChildrensRightsArticl es/fs/en?CONTENT_ID=4002945&chk=AaZA0p
Chris