It’s a classic money-maker, and for good reason. You get paid (well) to watch TV and not very much else!
Be warned that you will need a CRB (Criminal Records Bureau) check to look after small children, even though some parents may not ask for one. If you are wondering what to charge, look at local Gumtree ads, but you can expect to be paid over £8ph even if you aren’t trained in child care.
Aside from advertising yourself, you might want to try creating a profile on Findababysitter.com. It really is easy money (unless you get stuck with the child from hell!).