I'm going through this right now with our new nanny who is 24 and does not smoke.
She can buy on the federal exchange (healthcare.gov), but unless she has a qualifying reason, she won't be eligible for 2014 anymore. She can do open enrollment in November for January.
Another option is to go through SHOP. This is a small-business health insurance exchange. In addition to your premiums being tax-free, you would get a tax credit of about 35% of the premium for offering her insurance through SHOP. I found their prices to be about 35% higher than the Obamacare exchange, and there were fewer options, BUT it might be a good choice for you.
What we are going to do is she is going to get a short-term policy through ehealthinsurance.com, and then do the exchange for next year. These are not great policies, but they at least provide some coverage in the meantime.
Finally, I went to the exchange and looked at the plans (we are not in the DC area anymore). They all bounced around $200 for her age, so that is what I am giving her. If she wants the Platinum Plan, she can kick in another $100 for that on her own. I think a lot of employers do some kind of cost-share like most businesses do, though you'll want to talk to her about whether she'll take the insurance at all if her part of the premium is too high.