N.N.
Kid's parties are so hard these days with everybody so busy. When I have parties for my girls, I always over invite, that way when only 1/4 of the kids show up there are still plenty of kids there. I'm assuming since it was a sleepover that you know all of the parents personally. We only have sleepovers with girls that we know the families. I always call ahead when inviting to introduce myself and get to know them. My husband teaches where the girls go to school, so a lot of the parents know him. We also have the parents drop off, come in and stay for awhile for pizza or whatever so they are comfortable with leaving the kids. If they aren't we give a call when things are winding down and the kids are ready to go home. Sleep overs are hard these days. If it were me, I would call the parents next time maybe even before you send the invitations, just to warn them. You may get a better response with a personal invitation.