Is this for a planned stay? If so, ask some of these beforehand, as it seems to vary so much. We are very lucky – every bay has a camp bed, they provide bedding and towels for parents, and also feed one parent. I tend to sleep in leggings and t-shirt rather than actual pyjamas, but that's personal preference. Be prepared to be woken when they come in to do obs (and meds, if required overnight). They try to be quiet, keep it dark, etc., but certainly I wake anyhow, too tuned in to any noise coming from the direction of my child.
Be prepared either to entertain your child constantly or attempt to distract yourself constantly, or anywhere in between. DD2 wants to play even when she's very ill, so I turn up with colouring, jigsaws, busy books, picture books, peel-and-stick type crafting, sticker books, small plastic animals, etc. On the flip side, many of the children we've shared a room with have slept a lot or just wanted to zone out and watch a film, and then it's a lot of time to sit and worry.