You are basically intending to run your own business and will be viewed by Inland Revenue as self-employed - whether you like it or not! So sort out your stall in those terms ie what's the product, who is it for, what does it cost etc and use the many small business advice books and websites fo guidance. Make sure you are looking at recent UK advice, not US, in legal and tax terms. You need to think of the costs of starting up, overheads, and how much you want to earn in profit.
Especially if you intend dealing with minors, you mustobtain a personal CRB check. This can be difficult as mostly this is done on behalf of employees rather than by oneself - sometimes local business organisations may help you get it.
You will need insurance and will need to be clear about what you are insuring for.
Most libraries have extensive sections on starting a business and I recommend you use them.