I have an employee who I need to approach about her lack of work. How can I do this tactfully?

I work in a Day Care Center, and have an employee who sits around and watches the children misbehave, rather than deal with the situation. I'm not exactly sure how to approach her, as she may get really mad and just walk out for good. I need her as an employee, but she really has to do something other than just sit. Any ideas on how to talk to her in a tactful and professional way? Thank You!