Not to be a naysayer... really (ok, maybe)...but in most areas if you can not afford daycare for 2 kids, you certainly can not afford a Nanny. A nanny needs a salary to be paid that she can pay her bills on and live off of. As the employer, you need to pay payroll taxes and overtime if they work over 40 hours. Its the law. This in itself might cost you more in tax accounting fees for the year.
Sorry, but I get downright irritated when people who choose to have children, and must work, say they can not afford childcare. For us, that was part of the planning. Many people make sacrifices to afford childcare, or alter their life so childcare is not needed. But to attempt to hire a nanny who will not be able to live off of the salary you are likely proposing just gets me upset. Childcare providers of all kinds need to be able to live off of what they make as well. Its a career for many of us and needs to provide a living wage. Many of us are educated, trained and experienced in more ways than you can imagine. Your question disrespects my fellow childcare providers (of all kinds), even if that was not your intent.
I was a nanny for 8 years for a family of Physicians and have been a home childcare provider for almost 15 years. And you caught me on a very crabby day.