Just found this on the Directgov website:
The youngest age your child can work is 13 years old, with the exception of children involved in television, theatre, modelling or similar activities.
If a child is offered work in these areas, they will need to get a performance licence. Performance licences are issued by the local authority. Before granting a licence the local authority will liaise with the headteacher of the child's school to ensure that the child's education will not suffer should that licence be granted.