You have three options here, depending on your financial circumstances, how much you value this nanny, and what exactly she needs.
You can turn her down and say you can't afford to contribute. She may be able to find another solution or she may need to find another job.
You can offer her a monthly stipend, $150-200, in a separate check, that she can use to purchase her own insurance.
You can offer to cover her insurance plan - whether this means you take the time to sit down with her and pick one together, or if she already has one she wants to continue, this will probably be your most expensive option.