They happened on every PC and TA I worked (5 all thogether).
I can answer questions for you, but each one I have worked has been different but along the same lines.
The last few I did they were ticketed (free, like the character tickets) and they say they cap it at 100, but they do give out more as not every child ends up staying.
It's usually open to all kids 5 and over. Kids get a bag a day or two before to put their stuff in and instructions for the actual night (again, those have varied every time). They bring their own blankets and pillows from the cabin, your host can get more if you need, and they'll be sleeping on the floor.
Any self check priveledges are suspended from midnight until the next morning.
It is on the navigator, see coast to coast at the bottom, the event kinda finished at that end time for anyone not staying over, at about 11 they go have pizza then get ready for bed.