I would suggest you look carefully at HMRC: PAYE Basics, as depending on your circumstances and how much this job pays, your employer may need to operate PAYE and may not be aware of that. The NI LEL goes up as of 6th April 2011 (to £102 per week I think), so looking at www.mranchovy.com/calc/ if you work 10 hours a week at £10 gross per hour and your tax code is 746L then there is no Employee NI, no Employers NI, no Employee income tax.
So looks to me that £10 gross per hour for 10 hours of work would not mean your employer would need to operate PAYE... unless you have other income.
I don't know how old you are... don't want to offend but if you receive a Pension then that I believe is considered to be income. So I think that would affect how much you would be taxed in a job.
Not sure about state benefits, I think some are and some are not considered income. So if in receipt of any benefits, check how having a job would affect your benefits and if your employer pays below NI LEL would they need to operate PAYE or not.
With regard to the cost of having a nanny vs nursery - a nanny is usually MORE expensive. In your case I suspect that may still be the case, as salary wise alone you are looking at £100 a week and there are then additional costs involved such as admission costs for outings/activities, travel costs for outings/activities, cost of providing you with lunch (that may depend on if the contract provides for that or not), a small amount of cost for having lights/heat on at home during the day more than perhaps would be the case if no one was at home. If employer has to operate PAYE, then there are admin costs associated with that even if they do it all themselves - such as cost of phone call to a helpline, production of a payslip. If using a payroll agency, then that costs - varies, though I'd say £12-£13 per month is typical (there are cheaper and more expensive options).
Therefore if your potential future employer wants the costs to be lower than nursery... I think you will need to ask for a lower salary than £10 gross.