Being self employed means you can do any job that you are capable of eg.. bar work and beauty treatments and charging appropriately for those services.
You will need to show accounts for all you spend and all you earn and it is your choice if you have separate or combined accounts (I would say it will be easier to have separate for you to see which is making/losing money etc..).
Tax people do not care if you do 1 job, 2 jobs or 3 jobs as self employed so long as you declare all hours worked and you declare all income.
Remember that time spent keeping accounts, research, traveling to & from clients or suppliers, advertising (even speaking to people on the street about what you do counts), as well as actual time spent doing treatments/making things all count towards your hours so ensure you give yourself the credit due when asked how many hours you work.
I am not a single parent but I do claim working tax credits and this was the advice I was given by them when I asked if I could do bar work or typing as well as my beauty work to help bring money in.
The amount you will get from WTC will be based on what you expect to earn in a year. The amount you get from them will be the same if you earn £1000 from 1 self employed job or £1000 from 2/3 self employed jobs.
