I had the idea of building a summer camp for my sims kids, however, I had a few questions regarding children and vacations.

1) Can children go on vacation without family?
2) Can a non-related Teen/YA/Adult/Elder take a child on vacation? If so, can they take multiple children from different families?
3) Can children get their own room for vacation?

1) No.
2) Didn't try it, but seems positive. No, they can't take kids from different families.
3) You decide in which room the kids will sleep, after you make reservation for several rooms.

you can use simlogical meting controller items to set this up have an adult go on vacation there and start the meeting at the campsite it will summon whoever you choose and you can keep them there however long you want.