In your other thread you say she will be doing a couple of hours a week at the pre-school. A pre-school session may be 2.5 hours, might be 3 hours, probably isn't longer than that, so she isn't likely to be earning a lot if she is doing those hours. She may earn £1000 or less a year from that job.
I think that if the taxcode is split (see MrAnchovy's reply to you on other thread about splitting taxcode) and if she is earning £1000 from the other job, then £7 gross could still be £7 Net when doing 16 hours a week (taxcode 710L perhaps?). If she's earning £2000 from the other job, then £7 gross on 16 hours a week I think still becomes 7 Net if taxcode 610L. (Not sure that is how earnings in another job affect the taxcode.)
Thing is you don't know what taxcode will be allocated to your job and you don't know that the coding won't be changed at any point. It can be changed due to various reasons - repaying HMRC for underpayments for example.
You can change how much you pay them later on. The contract you agree now can give a figure such as £7 gross an hour and if you find that after tax codesplit have been agreed that your nannies take home pay has dropped a lot, then you could agree to increase their salary a little, say to £7.50 an hour Gross. You would try to give pay rises every now and then anyway, so giving one after the probationary period is something you could do.
You need to consider the maximum amount Gross you are prepared to pay and don't go above that. Employers NI I don't think starts to kick in until near £9 an hour based on 16 hours a week. So you don't have employers NI to consider at £7-£7.50 an hour salary. Thresholds for things like Employers NI can of course change, so as time goes by your costs will change depending on how UK taxation changes.
Have you called a nanny payroll company? They will often provide some help for free before you sign up to their service.