Slumber Parties sure take me back. I think us guys called them sleepovers because Slumber parties just sound inherently feminine. I don't have a good argument for that though, so don't challenge me on it. Anyways, I'm not sure if I ever had a big enough one to warrant their own invites, but I know others who certainly did.
Slumber Party invites aren't just paper telling the kids where to be, they're a sign to the parents that this is a party that will be well supervised and organized. As a parent, I'd feel a little more comfortable letting my kid sleepover somewhere if I got a printed invitation saying what all was going on and when to pick them up. It seems a little more formal, and therefor structured, and therefor safer.
So to maximize attendance to your event, get some Sleepover Party Invites, and we recommend from PurpleTrail. They're able to be customized without limit. We realize you'll want to change the wording and details, but you may not know that we allow for much more editing of that card than just texts or colors. Slumber Party Invites from PurpleTrail.