Can you put in a request to change the pre-school days or hours?
DD being at pre-school will not be saving you any money in terms of childcare cost. So it may be better if she went on a day when you are at home, thus giving you 1:1 time with DS.
Rural location, so someone coming to you has to live very locally or travel some distance - what distance would be practical, up to 10 miles? Do you have any major towns, larger villages within 10 mile distance?
A nanny is your only option I can see given that you say you live in a rural location and the pre-school is not on a school-run for a childminder. Are you sure that no childminders live near the pre-school? Something to check with HCC Family Info.
A nanny will expect a salary, given it only being two days a week (is that term time only, or would it be all year round?) then it may be at a higher salary that would be offered if you were offering a full time job.
Salaries vary, hard to give an idea but I would expect that if you were to offer somewhere between £9 and £12 an hour Gross, then you may get some interest - though it depends on finding someone within travel distance and for whom the two days fit around their other commitments.
You would be looking at:
7:30-6:30 plus 8am-5pm. So 20 hours per week. Having some over-lap between nanny arriving and DH leaving may be wise, so perhaps consider extending the hours a little if it suits you and the nanny. Perhaps call it 21 hours a week.
21 hours per week at a salary of £11 gross per hour, would be £231 gross per week on which there will be some employee income tax (the amount will depend on the employees taxcode) and employee national insurance. There is also Employers national insurance of around £11 a week.
So you will need to register with HMRC and you would need to do payroll (PAYE) deducting/paying taxes as required and paying those quarterly to HMRC. A nanny payroll company can assist with this, or you can do it yourself using HMRC Online, or payroll software. A nanny payroll company charges around £150 a year to do monthly payroll - they will also get you registered with HMRC as an employer and do the annual employers return plus remind you when to pay HMRC. www.payefornannies.co.uk is an example of a nanny payroll provider (they have been doing mine for 7 years now without issues).
Other costs to keep in mind are activity budget, food and drink for nanny whilst on duty (so lunch, afternoon tea eaten with your children), mileage if nanny uses their own car for taking your children out (such as taking DD to/from pre-school, going to the playground, going to swimming pool, going to a museum, to a baby/toddler group).