Thanks for the details. I'll see if I can persuade her to go that route!
I am trying to avoid a situation like I had almost 2 years ago with those 2 sisters and them not getting picked up until 2 in the afternoon. Emma is insistent on inviting the one sister who is in her class, they weren't in the same class last year but they are now and apparently they are glued to the hip. And the mom and I just don't talk like before I had Lucy. She is the mom who showed up last Oct (I hadn't seen/heard from her since school got out) with a gift for Lucy and then complained her mom backed out of watching her girls so her and her DH could go away.
As long as you are clear that the party will be over at such and such a time, and you are not available after that, you should be fine. My kids actually like the late nights better than sleep overs. They get all the fun, jammed into a short time, and there's really no time for arguments and fights.