Apparently there isn't a minimum wage for children under 16, but this is from Direct.gov
There are different levels of NMW, depending on your age and whether you are an apprentice. The current rates (from 1 October 2011) are:
£6.08 - the main rate for workers aged 21 and over
£4.98 - the 18-20 rate
£3.68 - the 16-17 rate for workers above school leaving age but under 18
£2.60 - the apprentice rate, for apprentices under 19 or 19 or over and in the first year of their apprenticeship
If you are of compulsory school age you are not entitled to the NMW. Some of your other employment rights are also different.