Hi, I'm Robert and I'll be helping you with your financial software question.
Basically the "nanny tax" is just added to your own tax burden and the interest and penalties would be from under paying your own tax ( with the nanny tax added ) If you don't pay it and the quarterly amount you pay for your own tax is not enough to cover the nanny tax as well, you will owe interest and penalties. Its probably best to estimate the tax owed and just add it to your own quarterly payments. If you are being taxed on payroll by your employer you might just drop a deduction ie if you were being withheld at 3 dependents ask them to withhold at 2 instead and you probably cover the difference and can sort it all out at the end of the year.
The federal unemployment tax can be paid at the end of the year. But make sure that you include it as it is .08 if paid on time and 6.8 % if not paid on time a difference of about 500 bucks on what would be less than 70 if paid on time.
This page has a brief description of what you need to pay