K.G.
My daughter is 9 and still wets at night....we use Good Nights for that. They come in all sizes and they even have them now that look like a pair of shorts. I checked with her doctor and he said there is nothing physically wrong with her...however she is a very tiny person...she's about the size of a 5 year old.
At that age my son also wet and his doctor put him on a medication called DDAVP. http://www.drugs.com/mtm/ddavp.html
Our doctor offered to put our daughter on it too, but we declined. He said she will out grow it, it might take some time. And for now, the Good Nights work for her. I just hate to put her on meds if we don't need to.
You should have her parents check with her doctor about it, if it's a big issue. But, until they get home, I would suggest getting the Good Nights..... it's better than wet sheets everyday, and the embarrassment that goes with it.
Also, make it very clear to the kids that they need not ask to go to the bathroom, they can just go when they need to.... let them know that they can feel at home at your house and can use the bathroom at any time.