S.E.
I would ask some friends and people you respect if they know anyone they can recommend. Your best bet is having someone that is related to your circle of family or friends. Calling an agency will be spendy, and cut the salary of the person you hire drastically.
For the amount of hours, and what you are looking for (full time care for an infant 10 hours daily, and a kindergartener for 3 to 5 of those hours)...that averages out to $5 an hour. If you find a nanny with those hours 7:30-5:30, chances are she/he will not have time to pick up a second job...and that is way less than minimum wage. I would wonder how they can survive on that income. I would also be worried that you get what you pay for because most nanny's would not take a job with a wage that they could not live on.
That range would be more appropriate for someone who does childcare and has other children they watch (IMO). To be frank, I think you may have a really hard time finding someone for that kind of money, and should check into some really great daycares. I am sure there are some in your area and you can ask around to help you find one.
Good luck!! Nothing harder than finding good childcare!