me and my two sisters went to many sleepovers and all of them were fine. that said, my parents knew our friends' parents, and there was always adult supervision.

safron's idea is a good one - maybe invite your elder one's friends over for her own sleepover? that way she won't miss out, and you can also keep an eye on things.

personally, i do think your kids miss out on a lot if they aren't allowed to go to sleepovers.