What are you actually after? A childminder who looks after the children at her house or a nanny to come to your house? If it's a regular job you'll need to become their employer. Do you know what hours/days you'll need at the moment?
I'm a nanny. I get £11.50 gross per hour in my employed job (part time). Though the rate would depend on the job, between £10-12ph gross is normal if Tinies ads are anything to go by. I've always earnt between that down here.
I do ad-hoc work too, I charge £8ph for babysitting after 7pm and a minimum of £11ph for work before 7pm, that depends on how far I'm going though and that goes through my self employed books.
A childminder charges per child per hour but can take on children from more than one family, they are also self-employed so dictate their own rates/holiday/contract which you agree to (its a service essentially).
A nanny usually works for one family (unless it's a nanny share) and they are employed by that family, having them be self-employed leaves you liable for a nice fine plus paying the tax and ni you should have paid originally whether she's paid it through her own books or not. I'm only repeating it as I've seen so many ads down here for "self-employed nannies".