When can we require employees to take fitness-for-duty tests?

Q. We have an employee who we believe is having frequent seizures. She has a history of seizures of which we are aware, but she indicated that it was a one-time event. She asked co-workers not to call 911 when she has an episode. Her job includes responsible for children who are three and four years old. I fear that her condition may cause her to be unable to supervise the children effectively. Can we request she see a Neurological physician for a check up and clearance for duty? -- A.G., Virginia