Playing "Shower"

My kids go to a great daycare. I feel very comfortable with the teaching style of the teachers. I'm even more happy with the very open layout of the facility. If there is a door, there is a big window in it. There are multiple teachers in the facility, constantly rotating classrooms, and there are parents coming and going at all times. The visibility helps set my mind at ease that there will be no funny business. Basically, I don't have to worry about sexual abuse.

However, when I picked up my kids today, I noticed two older girls, around 7 years old, coming out of a bathroom stall together whispering. I let the daycare worker know before I left.

At home, I asked Elise if she ever played with Casey, one of the girls I saw. She said yes and I proceeded to gently ask her what kind of things they did together. The more detail I asked for, the less Elise seemed willing to tell me. Finally, I asked her where they played and one of the places she listed was the "shower". I don't recall seeing a shower in the stall, so I asked her where the shower was and she said it was with the bathrooms. I asked her what they did in there and she said they took off their clothes to wash themselves. She quickly followed that with, "It's just pretend, Mom!"

I hugged her and asked her not to play in the shower with Casey anymore.

I feel rather sick about this. I'm not sure what to do. I guess I'm going to explain this to the daycare staff. At the least, I don't want Elise playing with Casey any more. I'm leaning toward asking them to have a talk with Casey about the inappropriateness of her play. I'm more than a little concerned why Casey came up with this game . . . What do you all think?