Even though she is 16, your child is still compulsory school age, so your local authority may have restrictions on when she can work - check the website of your local education authority under "child employment" (this is assuming you are in the UK). This may mean your daughter cannot work beyond a certain time on week nights, and there will be other restrictions on what she is allowed to do, e.g. round serving alcohol, using certain machinery.
Have you and your daughter got any idea how many hours she wants to/ is able to work? Once she is employed, there may be pressure on her to do extra hours. Make sure she knows how to say no, politely and assertively. She may also want to take time off during the GCSE exam period. It will be up to her to communicate with her employers, so make sure that she is confident to do this. E.g. if she needs to call in sick, she should do this herself, not expect you to do it. That said, it is a good idea for you to gen up on employment rights, in case there is something you/ she are unhappy about, you can signpost her to more information. Your local authority information on child employment is a starting point as is the CAB website (again assuming you're in the UK).
For my DS, working as a 16 year old and through sixth form was a positive experience. As well as earning money, he was learning life skills and how to relate to other adults. All of which are better to learn when you are younger and not dependent on your earnings. And having employed 15-16 year olds in a previous job, it is great to see them gain their confidence in the working environment.