You do not need to be a guest at the resort. There is a minimum number of hours. I think it is two but I am not completely sure. They will tell you when you make the reservation. I'm not sure what time they open, but I think it is maybe 4:00. There is no maximum that I am aware of but I think they close around midnight. I have always made a reservation. I don't know what the maximum number of kids is for the night, but there have always been a fair number there when we have gone.
I would also add that it is good to ask when they serve dinner. The last time we went, we dropped DS6 off around 6:15. Dinner was at 6:30 but because we had missed the ordering time, he wouldn't get his dinner until the later time which was around 8:30. He was okay, but he never eats that late so I was a little worried. I think he had a snack. The other two times we took him, we dropped him off closer to 5 or 5:30 so I hadn't even thought about it.
I've never tried in room babysitting but we've been happy with the Sand Castle Club at BC and Lilo's Playhouse at Poly. Those are the only two we've tried. It is nice to have a night out!