B.K.
Oh goodness.... WAY too young! I wouldn't let my daughter go. It seems like people are always trying to do things younger and younger these days. This one is crazy, and I wouldn't even consider it. Too many what ifs, and the girls are way too little.
You can let her go for awhile, and explain that you will be there to pick her up at bedtime. You don't need to explain, but if they ask you can just say "we don't do sleepovers yet!"