My daughter went last year. This year I booked it at my 180+10. However, last year I booked it no problem around 60 days out. It was not a full party when my daughter went in September. It's at 2 pm, selected days of the week. We are going on a Wednesday. You have to call, it is not bookable on the website.
Parents are not allowed in the room with the children. You can't stay and watch. You can sit on a sofa just outside the room in the lobby. A couple of parents never moved from the sofas near the entrance. Since it's only an hour total, I wandered around the grounds of the Grand Floridian since there isn't much you can do in the timespan of the party. You can immediately after the party go to the photopass person and view pictures of the party.