What Causes Pulling Down Pants By A 5 Years Old Girl?

I run a daycare and one of the little girls (five years old) I watch keeps pulling her pants down. It s ebough of an issue that she has been kicked out of other daycares. She knows it s wrong and that her private areas are for her to explore when she is alone. She only pulls her pants down for the boys and often tries to coax them into doing the same thing. I always catch the situation before the boys pull down their pants. How do I stop this without causing harm.